I hope you are able to. Ta-da and Burt were best beds her first 7 years with me , then Burt passed sbd she had Neptune, when Neptune was lost she has had no one fir several years. Abd seeing the other green cheeks together, did make we want that again for her . If I could find an older one needing a home as Ta-da is getting up there..
Do keep in mind not all will be friends. We have green cheeks seriously maime and even kill other green cheeks. ...they can be fierce. Odds are better when added as young ones or adding a young one with your older , but not Guaranteed. I do think if taken slow , giving each other a chance to get used to another in the room and being very careful when you do first introduce in a neutral area to not allow squabble at very first meeting increases success.
I was careful, tho it it went fast , to base on both birds behavior and comfort level and I did do at least a hundred walk over with one to the others cage and treat both and switch who I did with. And I did sit with one next to other cage and just chill. Never allowed one to land in the ones caged while they inside. I do feel comfortable reading and interpret behavior and cues