I have two males living happily together in the same cage. They are about 1 1/2 years old now. Here's why I think it works.
1) Although from different clutches, they came from the same breeder and were caged together as soon as they had fledged, so they knew each other before I brought them home.
2) I don't think either one has a severely aggressive personality - just average or even low average.
3) They live together in a large flight cage.
4) They're fully flighted so that they can get away from one another before a little squabble turns into a big squabble.
5) They each have their own food bowls, and I actually put the bowls in the cage "at the same time," so neither is tempted to start eating in the other's bowl. (Really, the main reason for any squabbling.)
6) They are semi-tame (get on my shoulders, land on my hand, eat off of it, fly to the side of the cage when I walk into the room and start chattering to me), but neither has ever been "spoiled" by me so they don't fight over my attention.
It works and I think the quality of their lives together is worth the risk that one would injure the other, although should I ever notice that they've started fighting or that one is getting very aggressive, I would most certainly separate them For now, they are adorable as they play together, preen each other, and sleep snuggled together.
(I had multiple linnies living together in a double flight cage at one time, and some were quite peaceful, a couple were average, and one was extremely aggressive - towards the other birds and me. Needless to say, I had to move him. He was the most aggressive bird I've ever had, and I've had a few over the last few decades.
So...the bird's individual personalities play a big part, imo.)
Just remember that it is a risk. I know of parrotlets that have killed their partners or cage mates, so you have to believe that their quality of life together is worth the risk. In our particular case, I believe it is.