the European rule of thumb is to offer about 8 shell lengths by 4 shell lengths for one, and 8x8 for two or three. These guys hit 12", so about 8x8 for two.
However... there is a lot of wiggle room here. Red-footed torts are a little more tolerant of crowding than many other species, and they are also less territorial and aggressive. Also, if they have a nice, big outdoor space in the summer, then a little less space in the winter is not as big a deal as it otherwise would be.
So, you could probably do the two adults (and it takes several years to get to adult size) in an 8x4 or even a little less. I know a lot of keepers consider 6x3 to be a large, spacious enclosure.