Well, I'm not sure of your layout, but I'm guessing there's one common wall. If the the 6' wall is the common wall, just design it for one tank full and make sure you have the critical reinforcing on both sides. Corners and the base are almost always critical. The exterior walls are designed independently.
If the common wall is the irregular sides, you'll have to check each tank as full and make sure the corners and bases are reinforced for both cases. You might be able to use the shorter wall to support the longer wall.
But with those dimensions, if you use a 12 inch wall with #6's each way, each face, (minimum bending reinforcing) it's going to work.