While one shower is in use, the other receives only cold water.

[John.g]

Have you tried just throttling down the flow on both showers to see if you can get both to deliver hot water together, for the odd times that you require both on it might be relatively easy to get a feel for the position of the flow control, it (they) might even be indexed, obviously you will only get "half flow" from each and would require the cooperation of both users!.