introducing adult males

Question
Hi there,
I have found myself in a unique situation.  I volunteer at the local human society.  I recently adopted an adult rat from there.  I've had rats in the past, but only singles, and after reading more about rats and their behaviour, it sounds like they are happier when they have a friend.  So I went back to the humane society to adopt another adult male, the reason I chose him was because they had come in the shelter together and used to be in the same cage.  They separated them because they didn't have a big enough cage to keep two rats in together.  So they have been separated for about a month.  I tried introducing them on neutral ground and everything seemed fine, they even started grooming each other almost immediately.  Then I put them in a nice clean brand new cage together.  Everything was good...at first.  There was a bit of fighting, and some grooming going on.  But then one of them tried to mount the other one a couple of times and at first the other just shook it off.  But then for some reason, major fighting and pushing and squeaking broke out.  I separated them again.  So should I just keep introducing them on neutral ground until they get used to one another again?  
Thank you
Ade

Answer
Oh boy....it can be really hard to introduce adult males esp if they have issues with aggression due to poor breeding.  Sounds like they are simply trying to decide who is alpha of the cage,so yes, I would keep trying on neutral ground.  Here is a good article and introducing rats, although you have done it right, there may be some tips  that are handy.
Good luck!

http://ratguide.com/care/behavior/introducing_rats.php