I would definitely go with what megcornell says and buy another tank (at least 75 gallons so it may be your last upgrade, therefore saving you money in the long run) and keep the other turtle in the 50 gallon until you can afford another set up. Even if they did get along, you would need at least a 120 gallon tank, but since they don't, two separate set-ups is the best way to go.
You definitely want to strongly consider re-homing one of the turtles if you can't afford 2 set ups.
Try to look around on craigslist to see if you can find a large cheap aquarium (they're rare but they're there).
