If it is just your hot water pressure lacking and these showerbodies are off the same riser I would look for a partially closed valve.
If the pressure coming out of the H/W tank is good to other fixtures it would be the riser supplying these showers is galvanized piping with lots of internal corrosion.
Was this a chronic problem or did it start slowly?
Has anyone performed any work in your home prior to this condition happening?
It could also be a defective dip tube BUT that would also effect your other hot water applications.
How is your other faucets working H&C?