I do not have two rotti's but I have a rotti and a pitty and Bronx, the rotti was about 1 yr 3 mos when we added on Brookie, she is now about 10 mos, and yes we are most likely getting a rescue pitty soon, in the next few weeks assuming all goes thru.
Bronx was great with the puppy, mind you we had to keep an out for him crushing her, but they are best of buds, but he was very well socialized with other dogs so he loves em. I am hoping the new addition will go just as smooth, Brookie also loves other dogs, she is more of a suck then him, did not think it was possible. we just make sure to give them equal attention and buy two of everything, soon three of everything I guess.
I guess all dogs act differantly towards additions, but I wish you luck if you get another...its hard work, but its worth it...In my opinion, but I am a dog nut.