Here is what Dennis Quinter has learned over the years:
1. Two males may or may not get along. (siblings do better, but two
unrelated males can get along)
2. A male and female may not get along. (rare)
3. A male and female does not guarantee there will be babies.
4. Two flyers can bond together and be happy without their human
if they are not both well bonded to humans before being put
5. It all depends on the flyers involved and they are all different.
Introduce a newcomer slowly. Put cages side by side and touching, to start, for about 2 weeks. Swap some of their bedding material so each can get used to the other's smell. Trade food bowls every other day or so. Too frequently and they might just start skipping a meal.
Before allowing much contact without a cage, make sure the pair are about the same size, in weight. (In other words, don't put a juvenile right in with an adult. Wait until the juvenile is about adult size... 12 weeks) If the youngster is too young when you cage them together she may bond to him instead of you. Make sure there is at least two nest boxes, three would be better before putting them in the same cage. That way they can each have their own nest box and still have one to store food.
Allow an older flyer to access the younger's cage and crawl on the outside. The younger can approach at will, but is protected inside the cage.
Gradually introduce mutual shirt time so you can remove one if necessary. Flyers will "hump" each other in play and establishing dominance, but it is easy to tell real aggression.
Out of cage mutual play time (under strict supervision) is next. Tag, bear hugs, humping and chasing are all part of flyer play. Even smaller flyers will let it be know when they've had enough from an older/bigger flyer.
Roughness to injury is rare ... although they do seem to have an affinity with attacking each others tails sometimes to the point of slight bleeding and splitting of the tip.
An existing flyer seldom rejects a new companion. They also do not rejected their HOF for bringing another flyer into the house. If they are very near the same age, and very young, it should be easy to get them acquainted. Just let them get to know each other at play time out of the cage. If one is much younger a little more time should be taken to get them acquainted. Use the separate cage method ... then put both in one freshly cleaned cage so neither can claim it as home territory by scent. Make sure there is more nest boxes than squirrels so they can get away from each other if they want. There may be a few scuffles and some minor squabbles, but normally nothing serious as they generally prefer to have company.
Judy C's recommendations:
I have found that the best way to get two of them together is to put them in an entirely different cage. That way, no one is 'territorial'.
If you don't have access to a new cage, try moving everyone out of the cage you want to use. Then wash it really good, use bleach, leave it in the sunshine a few days if possible. Redecorate it completely - different perches, toys, everything. If possible, put it in a different location. Then introduce them both to it.
Good luck! It worked for me - I hope it does for you.
Tom Hauk's Squirrel Match Maker Method:
Set up a new cage that neither has been in. Transfer them from their old cages to the new cage where there are several new houses (at least 3) and nothing that either had been using before. Put them in at different times and give them each a house. Leave them alone for an hour and then take each one out for shirt time and just set them in front of a fresh bowl of fruit and a few meal worms when you put them back in. Don't be surprised if they both end up in the same house and sleeping soundly, tightly up against each other.
I have introduced babies old enough for the 'big kids cage' by letting the new baby explore the cage (and introduce his scent) while the big kids are out or sleeping. They all get to know each other during out of cage play before this. Then one day I just pop new flyer into the big cage during the daytime while he is sleeping in his own nestbox and the big kids are in their boxes. I keep watch, especially when everyone wakes up at night. For the first several nights I let everyone out so that all are on nutral territory to play. Leave the cage doors open so everyone can come and go, and explore each others nest boxes and stashes.
Oh, and I hand out lots of new in-shell nuts during this period. Nothing seems to bond squirrelies better than working together to stash nuts.