Co-existance of two pair of perculas?
 

I am thinking of keeping adding a pair of true percs to an existing pair of black percs, tank is 120 gallon size.

Anyone has done it before or has any advice??

Well According to clownfish by joyce D wilkerson Perc have been known to fend off maroons. So unless you are willing to tear down your rock structure looking for these 2 fish that are getting the crap beat out of them i wouldnt. On the other hand if your one pair is on one side of the tank and your new pair is on the other with a new host i think it might be fine.