I would say that any situation in which any leading FRFR system doesn't provide enough volume is a situation in which permanant damage is being done to one's auditory system. All of these systems easily produce output levels which far exceed the a safe time/intensity noise dose per OSHA standards. In other words, mandates state an individual can not be exposed to these type of intensity levels as part of their job duties without the use of hearing protection. Well, for many musicians, music is in fact their job, and they are in fact doing significant harm to their auditory system which can be expected to result in conditions such as hearing loss and tinnitus.
Instead of trying to add a second FRFR system to compete with excessively loud band mates, why not instead have everyone else turn down ?
Granted noise induced hearing loss is good for my business as an audiologist, however, I also see quite a few musicians who are now extremely bothered by things like constant tinnitus and whom attribute it directly to high stage and rehearsal volumes. They really wish looking that they took more precautions to protect their hearing.
2 monitors is awesome for enjoying the stereo spread of some Axe patches, but 2 just to get louder, when one already shakes the walls, is in my professional opinion harmful.