Tiels are agressive should I separate them?

Question
Hi, I have two tiels and was told they were both male. Now I'm not so sure we have had them for 2 years and the yellow one Nanners is very quiet and the gray one Mayo talks constantly he also mounts Nanners and fights her for food. Until recently they were very gentle but now Mayo doesn't allow anyone near the cage and if he falls off he will bite you when trying to pick him up, and Nanners hisses anytime you get near the cage. No eggs as of yet and it has been almost a year since he started mounting. So should I separate them? They are very bonded and scream when they cant see the other one. Any advice would be great! I've lost enough blood :( Thank you for your time!

Answer
--  Here's a curious thing about birds in captivity:  When two birds are housed together, even if they're the same sex, they may display 'mating pair' behavior.   One will assume the role of male, one will be the female and they will behave accordingly.

 Sometimes it's from stress due to a 'too small' cage.  How to judge cage size:  Imagine that both birds are on the same perch stretched across the center of the cage.  Now imagine that they both, at the same time, fully extend their wings and fan out their tails.   Next, with wings fully outstretched they completely turn around

Neither bird should come close to touching each other or any part of the cage with any of their feathers.   That's a big enough cage.

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As for housing two birds together - other than finches, canaries, doves and the other 'straight bills' - I would never do that.   All birds here (all parrots/hook bills) are in their own cages.

This way they remain bonded to the human first and each other second

You can see more here  http://www.4animalcare.org/birds