Pairs of cards read together is a skill you don't necessarily have to learn to read tarot, however many readers want to know how to relate the cards to each other. Some spreads naturally have pairs of cards together, too, so if you want to learn how to use those you're going to need to know how to see the two cards interacting with one another. I often use a Double Celtic Cross, so I've become pretty comfortable with card pairing. Reading pairs also comes into play if you use clarifiers.
To clarify or not to clarify, that is the question. Some readers say no way, clarifying cards confuse rather than enhance meanings. Others, like me, say wellllll....not always. Sometimes, especially if you are unsure about the meaning of the first card, they can muddy the waters further, but it depends on how you use them. You knew you wouldn't get a straight answer out of me, didn't you? I'm all about the "well, it depends..." when it comes to tarot.
For those who don't even know what I'm talking about, clarifying cards are cards laid on top of an existing card in a spread in order to double-check or enhance the meaning of the first card. I find their best use is when you know what the first card means but want to know more or you want to be sure you've got the right aspect of the card. For example, if The High Priestess comes up and I'm feeling the card is stressing the secretive side of the Priestess rather than say, the passive feminine side, I may draw another card asking if I'm on the right track. If say, the 7 of swords clarifies it, BAM! I'm on the money. If the 4 of pentacles clarifies it instead I can change my track and go with the more passive, withdrawn meaning. Other times clarifiers can be used is when one may desire more information. I like to pull another card when, say, the Tower shows up in order to see what the result of the shake up will be. Another card that is useful to clarify is Death to indicate what may come afterwards, what the situation is transforming into. They aren't always necessary or desired, depending on your own intuitive sense about the card and the reading as a whole. Just remember, when using a clarifier, the first card is still provides the main message and should hold the most weight. The clarifier is meant to add some depth but not be read with the same weight as the first.
This isn't so for spreads that use pairs in each position, such as the Double Celtic Cross. In those spreads, each card in each pair holds equal weight so the interpretation comes from a blending of the two. This is where it can get a little difficult, depending on the pairing up of the cards. It helps to begin thinking outside the box of "good" versus "bad" cards because if you think along those lines and get one of each in a pair you're going to have trouble right off the bat thinking they are negating each other. I'm going to give some examples here because actually reading the cards is really the only way to demonstrate what I mean.
First, here is an example of using a clarifier card:
The question is about a situation with a co-worker that has the seeker concerned about the possibility it could endanger her stability at work. This three card spread's position meanings are Situation/Advice/Outcome.
So the situation, Temperance, shows that the situation with the co-worker will take time and patience to work through, but it's not as dire as the seeker may believe. They may have opposite views on this situation, but it can be worked through. They are attempting to find a middle ground between them, and given time they should be able to find an equitable solution. Certainly this is what the seeker hopes anyway.
The advice, 2 of Wands, tells me that the seeker needs to focus on her own work primarily and not allow this interpersonal problem with the co-worker distract her too much. If it appears the situation could indeed threaten her job, she should put out some feelers to other places to work. She needs to examine her options and choose her course of action.
The outcome card, 9 of Wands, seemed to be saying that she will find herself a bit hemmed in by this situation, but I wasn't sure if the conflict would be resolved or not. Given that it is a nine, I do see an ending, but it looks like it's going to leave the seeker a lot more reserved and cautious around the co-worker. I also see that her job status will be protected, but I was a little concerned about the woundedness of the figure and how it may affect the seeker's job performance, so I clarified the last card. The 7 of Wands shows that all in all, the seeker would successfully overcome in this situation, she would prevail, though it won't be easy. She will need to stand her ground on some very crucial points and compromise on others, but in order to protect herself she will have to stand firm. The two cards together show a cautious but successful attempt to work things out in a way that benefits the seeker.
For pairs of cards, the interpretation is similar, but the cards interact in a slightly different way. Neither card holds more weight than the other, unless one is a Major and the other a Minor, but even then you have to see how they blend. For example, here we have the Tower and the 4 of Pentacles.
Depending on the position meaning, the cards could be saying several different things. Initially, the Tower is the stronger card given its intensity and the fact that it is a Major. With the 4 of Pentacles solidity, though, I get the sense that while the Tower will shake things up quite a bit, there will be a solid core that will not be shaken. There will be something left to hold on to throughout the Tower experience and afterwards as well. It's as if the cards are saying, hold on, let things quake around you, but you will be able to hold centered throughout, just keep it together. You may see something different. I can also see several different interpretations depending on whether these cards are advice, prediction, past, or whatever. This combination could also indicate that while the Tower will shake things up in several areas of the person's life, their finances will not be affected, per the 4 of pentacles. Or it could be saying the reason the Tower is erupting is because the person has been holding on to something too tightly and has created a kind of pressure-cooker which results in a Tower experience. See, it all depends on how the cards fall in the reading and which way your intuition is telling you to go with them.
I don't use elemental dignities, but they can also be used when interpreting pairs. When you have opposing elements, or elements that strengthen or weaken each other, you can bring that, too, into play. However you choose to interpret the cards, it's easiest if you don't necessarily view each one separately then try to blend them, but look at them as a unit to begin with and see how the messages of the cards interact and blend. Place elements of each card in the other card. For example, you can envision the man in the 4 of Pentacles sitting fast in the Tower basement, debris falling round him, but he remains guarding his treasure. Blending the elements of the images like this will help you feel out the meaning rather than trying to interpret them individually then try to connect them somehow.
I use pairs of cards a lot in my readings, but I wasn't always comfortable with the practice. It's something I've grown into and become more fluent with, so if it's not for you, don't worry about it. It's just one more way to read the cards.