its not that easy... sure you can chosse maps like muttrah alt or maps that dont start at the water but you have also to change the spawnpoints and movment of the bots. another thing is that the bots have different vehicles. air assets dont need a warmup, they are autorearming without crates or helipads, bots dont need a crewman-kit...
so its not that easy. on the other hand you dont need another server for this because if you got templates and maps you can just restart the server and load another maplist. if im not mistaken.
if we have two servers for coop the mainproblem is that we divide the serverpopulation. and opfor with 12 player should be very hard.
and finally what m8 already said. Many players don't want to play 5 maps in a row without a scope and tend to feel stressed. opfor is similar to some nam maps such as ghost train you start with 35 players and at the end there are 16 left. and then you can only win via tickets, which takes three hours...