whisper

hi all im new to this forum , i am currently owned by 4 african greys , 3 are timnehs and 1 a congo 2 of my timnehs paired about 6 months ago and have become inseperable i have noticed over the last few weeks one of their heads is becoming balder on a daily basis , until yesterday had not witnessed the preening by the other one , they are also regurging to eachother alot , yesterday i put in a nesting box , WAS THAT THE RIGHT THING TO DO they are house birds and have a very large cage , although both of them are not friendly birds one is 15 and the other is 8 is it possoble they are trying to mate ???? any advice would be fab many thanks