It is not just tank volume that you have to consider, but also surface area, water flow rate, etc.
You can house 2 fancies in a 20 gal if you have adaquate water circulation. I would overfilter the set up (aim for something like 200 gph) to ensure adaquate oxygenation. A shallower tank (with bigger surface area) would also be better although I don't know if they make those in a 20. <These are called breeder tanks & I think the smallest standard size is 40.>
But even in a standard 20 gal setup, you should be able to keep the pair for years before you need to upgrade. <Just start with the 1 or 2" juvies & you can prob keep them till they are 4 or 5".> However, with smaller tank, you would need to do more monitoring & water changes to keep up with the water parameters. A bigger tank is generally less work & more forgiving of mishaps.