When one commode is flushed, it drains the available water for flushing in the other commode, which, in turn, reduces the water available to flush in that commode. Any solution?

Pure and simple! You have a blocked vent that must be snaked. Vents are there to prevent such things from happening, Vents relieve the suction caused by drain and flushing fixtures. When a vent's blocked the suction has to relieve itself somewhere, Some time a fixtures trap. In your case the suction pull the water out of your bowl.
The solution would be to snake the vents on both bathrooms. Good luck, Tom

If you have two toilets installed back-to-back than it should have only 1 (one) - 2" diameter common vent. Do as Tom suggested. Milo

Not if both toilets are wet vented by the vanity vents Milo, You're thinking UPC again. Cheers, Tom