How many tarot decks do you need? One should be enough. Or maybe two, an RWS and a TdM. Perhaps three, if you want a Thoth. What about a deck for different seasons, something bright for summer and dark for Halloween. If you read professionally, you might want to keep a separate deck for your personal readings and a selection for use with your clients. I could go on but once you start collecting tarot, you can always find a good reason for a new deck.
So if you have a collection of decks to choose from, you might want to use two decks at the same time. That will enable you to choose a clarifying card or a significator from a separate deck to the one you're reading from. Or you might want to use a two deck spread, choosing cards consciously from one deck to represent an aspect of your query and then pulling at random from a second deck to represent your subconscious (or the universe) at work.
Here are some deck combinations that I think work well together. To show their compatibility, I've picked the card in tarot that I believe represents a partnership better than any other and that is the 2 of Cups. As always, if you're choosing a new deck, pick the one you love.
So that's a dozen combos for you to consider. I love doing a two deck reading. I find the comparison between the cards I choose consciously and subconsciously can say as much as the individual meanings of the cards themselves.
If you are looking to purchase a new deck, here are the best places to head:
The Wootique https://thewootique.com
Little Red Tarot littleredtarot.com
Celia Melesville celiamelesville.com
Darkness Tarot darknesstarot.com
Three Trees Tarot www.threetreestarot.co.uk
Baba Studios uk.baba-store.com