Well i bonded 2 of mine, but had to seperate them again...
I had a female and male siblings, they got a virus and the male died, but the female survived. So i went looking for another bun to keep mine company, i made sure the virus was dead, left it 4 times the length of how long the vet said its active (she said 10 days, but recommended 20 to be safe, i left it about 40 after my bun was better, so about 56 days after the contracted it).
I got a male, bonded them fine, but then some people on here said he may be fertile, and as she wasnt spayed yet i didnt want litters, so after only 2 weeks of being together i had to seperate them, which in my opinion is worse for them than if i didnt introduce them! I bring them near each other everyday with a close eye so no humping, but i feel so bad!
If you want to bond them now, then get the same sex so no baby accidents! I read opposite sexes tend to easier to bond, not always the case, which is why i opted for another male, but most people find it fine to bond to males or two females! Infact many people, i have heard two males can be more tempremental but i dont know whether this is true, someone else could offer you advice on this.
Im going to rebond my two little ones after theyv both been spayed/neutored, but if you decide to go for opposite sexes, i personally would now wait until theyve been done! Saying that, both the breeder and vets said mine should be fine together, they insisted he wouldnt get her pregnant as his testes havnt dropped, but someone on here said they can be...so im not 100% sure, but i know id rather keep them seperate longer than have lots of babies!
Oh and age, doesnt matter really, but obviously if they are similar ages then theyre more likely to "go" together rather than leaving one behind. Just make sure to get BOTH buns done no matter what sex, it helps behaviour and stops cancer!