Yes plecos are territorial but if you introduce then together into a new tank at the same time, they would not fight as each have equal opportunities to mark out their territory.
pleco aggression are often overhyped. They will be territorial and will fight to defend or mark an area as territory esp when newly introduced to a tank. But once equilibrium is established, then things should be fine. You will of cos still see some occasional bites or scuffles but nuttin serious generally. Unless u get one lunatic pleco dat is bent on wrecking havoc of cos