Depending on the sex and the bird. I have realized when I put my male and female cockatiels in the same cage they became more aggressive towards me. I dont know if it is a sex thing or if it is just having some one besides from you in the cage. i would ask your local bird store or other pet store

It really depends on the birds themselves...not neccesarily based just on sex. Two females or a female and a male are more likely to get along than two males, but there is always the possibility. The only way to really know is to test to see if the birds are compatible, which you will usually discover in the first couple of hours. Put the birds in cages next to each other or let them hang around each other for awhile outside of the cages **Note: WATCH THEM LIKE A HAWK!!! Keep some thick welding gloves and a couple of large towels (to cover the birds with and use to pick them up in case they fight) If the birds do begin to fight, you need to separate them immediately because they have the capacity to and will harm each other--throw a towel over one and use the welding gloves to pick it up--it may mistake you for the bird it is trying to bite. If they hang out together outside of the cage and seem to get along okay, put them in the cage together--but make sure the cage has plenty of room for both. If you put them in a cage that is too small, they will fight like grumpy roommates crammed into a small dorm room. If they seem okay and it has been a couple of hours, it should be okay for them to be together in the same cage. **Do not attempt to put the birds together unless you have the time to monitor them immediately--there have been birds that have had their beaks and such ripped off by other birds, so it is absolutely vital to monitor them during this stage of them getting used to each other. Good luck ! :)