Yes, mine actually prefer to have a roomie. Just be careful that you keep an eye on them to make sure they are happy with each other in close quarters. Sometimes when extra stressed, like at a show and such, you'll see tension between dogs when it doesn't normally exist. There is definitely enough room to double up in the 300. Although it's not ideal, it's good for traveling and such. Usually when we're on the road they're zonked out anyways, and they like to lay on top of each other. They don't really need a bunch of space. lol
Sounds like your set up will work just fine. Good luck, and looking forward to hearing about the new little one!