What "can be done" and what is optimal for the health and longevity of the animal are often 2 different things. Fossorial animals should be offered a moisture gradient which IMO is more readily acheived with a "long" enclosure, with a piece of wood or brick used at the higher end. A 20 gallon long provides this and can easily accomodate 6" of substrate....adult tigers can acheive ST lengths of 12-13".
Retreating to water can be a sign of stress in the adult tiger - often associated with poor environmental conditions...too dry, too hot, fouled substrate, over crowding, etc. If you find your tigers retreating to their water dishes with frequency, their environment should be checked.
In response to the original question - yes, a 20 gallon tank can house 2 tigers. Would a 20 gallon long be preferable - I would say yes.