TWO turtles are going to be TWICE the work, regardless of whether they are in the same tank or not. Reason is: if you have them together, they will poop in that SAME water TWICE as much which means TWICE as many water changes/filter changes, etc.
Honestly just keep them separated. If you didn't want the additional hassle of "twice" as much work, then having only 1 turtle would solve that problem. Not trying to be mean here, just trying to let you know what you are in for in the long run.
