My boy, Oscar (desexed) was 4 1/2 when I got female blonde bombshell, Saffie (also desexed). He was a little miffed for the first few days but now they are inseparable. But as Margoo says, play dates are now a little more complicated. Oscar was always the best, friendliest dog around other dogs but since Saffie joined the pack he has become a lot more snarky - any dog smaller than him is allowed to play with the young blonde, but any dog his size or bigger is definitely not allowed. He doesn't fight, just an annoyed snap at both the other dog and poor Saffie for being such a flirt. That's all quite manageable for dogs who respect being disciplined by a cranky elder, but I always worry he'll snap at one who doesn't have such good "back off" manners. These days at the off leash beach, I put Oscar on the leash if I see a bunch of dogs ahead who look a bit OTT. He hates those types that rush in with no manners, whereas before he was much more tolerant.
Having never had 2 dogs before, 1 thing I do find is that your chances of disagreements with dogs you meet out and about sort of increases. The more dogs in the mix the more likely someone will not get on I guess. A friend I go walking with has 2 cattle dog crosses, brothers from the same litter, and they are best of mates - both desexed. Though it is interesting to see that one of them seems to take on the "female" role in the relationship (and no, by that I don't mean maxing out the credit card, more being submissive and the peacemaker). Oscar gets cranky with both of them, but thankfully they observe his "lord of the universe" status despite being much stronger and tougher than a show pony Border Collie!
Good luck with those big girl stunners!