No way to tell what the issue is without more info, and a pressure balance loop may not solve your problem. A typical 1/2" shower valve can supply in the order of 5-6gpm (you results may vary). A head is limited by Federal regs to a max of 2.5gpm. So, you could be exceeding the capacity of the supply lines through that valve into your shower plumbing. If you have pex in the wall for between the showerheads, that in itself may be the problem, and could be as well, if that is what was used to supply the shower valve, or possibly cpvc...both have a smaller ID than copper, and can carry less volume.
Basic shower info...the supply lines must be able to supply at least, and preferably more, volume than the heads were designed for...any less, and the performance will suffer. FWIW, it's very likely a volume issue, and unlikely a pressure issue, and it may not improve unless you can increase the volume.