How to stop african gray fron plucking feathers?

ive got 2 african gray parrots i recenty got,,,both were plucked when i got them,,got the male first,,,he was plucked on his chest,,,,then i found a female,,,she had been plucking her back,,,although since ive had the 2 together,,in seperate cages right next to eachother,,,i have not seen them plucking,,,,but I was wondering if any one out there,new of some tricks,,,or help to stop feather plucking,,,,thank you