Even though I am married an have kids. I am the one who basically does everything with the babies. I take them to and from groomers. Yes it is an additional cost but really not that Much more..
I will say that I don't feel as bad leaving them alone because they DO have each other. THey will play together, which I love to watch. They sleep together. As for flying, if you do go with a friend I know continental allow 2 dogs in coach. But only 1 in first class. So if you went with a friend you can each have one.
I really admire you for thinking of all aspects, not just the pros before getting another baby. If more people thought like you our shelters wouldn't be so full.
Best of luck in whatever your decision!!!