If you're talking about a rack or stack of identical cages (boaphiles, Animal Plastics cages, etc) that are all set up the same, then yes, you can use one thermostat. I built myself a 4ftx2ftx6ft wood enclosure that is divided into 3 4x2x2 cages (plus storage space) and I run all three off of the same t-stat. I put the temp probe in the middle one, I get the average temp that way. there is a couple degree variation, but only a couple degrees, in the top and bottom cages.
For two completely seperate tanks, theoretically, if they were side by side, set up the same, I suppose you could just use one T-stat, but I would not feel comfortable doing it that way. T-stats aren't that expensive.