44' Gulfstar '86 T diesel motor yacht charging system

With both engines running, starboard altinator and tack stop working, if you turn the port engine off, starboard alninator and tack work, turn port on starborad alt. and tack stop. Both main and genset engines all start from one comon starting battery. Can you tell me the solution or where to go to to have it corrected?

Thanks

Gary Hunt
ghuntwfy@aol.com