I have 2 pups atm, very close in age & still very young.
All I can say is you get twice the amount of UPS & DOWNS.
You need to be willing to put in twice the effort, double the training, double the playing, double the affection etc. Luckily for me my partner is alot of help (especially when they both start their obediance training classes soon)
I can't really comment too much as I have only had my 2 a short time and at the moment they are very stressing at times, but are getting better slowly. Alot of egging each other on, rough play & chasing etc. BUT they also share a snuggle afterwards on an old childrens lounge. My partner had alot of progress with them today. She left the backdoor open and only left 1 crate open for them. The younger dog (who soils every part of the house atm) followed the older 1 who is housetrained) outside to the toilet, they played for awhile then came inside and shared a crate for a bit. The result was 2 dogs that seem more bonded and alot calmer this afternoon/tonight. We usually crate them seperately but with the younger pup whining all night we are going to crate them together tonight and see what happens. If they play games all night we will seperate again but if they settle down for the night we may keep them together.
I guess IMHO what I have experienced in my very short time having 2 pups together is that its a nightmare to begin with, but I am sure once you have established what is & is not acceptable, and that the dogs needs are met individually as well as shared it will be twice the pleasure of any single dog ownership...