How to know when to pair cards in tarot?

@nisaba: Thanks for the great spread! Sounds fascinating.
As for me, I like to do the Opening Of The Key spread, which pairs cards also as part of the original layout (instructions available for example at supertarot.co.uk, but there are different versions around... on the net as well as in the books).

Doing positional spreads, I have no strict method when it comes to pairs. Sometimes it feels just naturally, that the situation described by one card, gives the explanation, how another card is to be read in that situation. It´s not easy to explain, it has completely to do with intuition. I would just try to get the meaning right, if there is the need to pair the cards, just do it! Comes naturally after some time.

Inutition.

I cross or pair cards constantly. I have an intuitive feeling that urges me to pull another card. But I am a psychic who uses cards. I am not primarily a tarot reader. I am mainly clairvoyant.

The easiest way to get in tune with your inutitive place is to practice meditation. You can do simple meditations with your cards. Count down from 10 in your head. Each time you breath in see a new number. 10 will be red. Breath out and slowly visualize the red turning to another red color or orange. Breathe in and see 9 in orange...Do this until you get to 1 or 0. You'll be in a much more relaxed state.

Or just breath and focus on the image of one card. Think of a mantra, such as, "Divine Source, help guide me. Divine source, help guide me." You will feel yourself in a different place. That is the place where intuition lays. You'll need to practice.

Once you are in tune with your intuition, pairing cards will feel right.