1. I feel for both you and your BF -- I'm constantly torn between forging ahead and second thoughts. I love Nutmeg and Romie to pieces, but our life was getting just a tad bit (lean in close here -- I'll have to whisper this) boring. What I said to a doubting friend (admittedly not as important in status as your BF) when I was considering adopting dog #3 was that sometimes I just had to go ahead and do something to change up my life, even if on paper it didn't look like a smart idea. That argument seemed to sway her.
2. I think walking two dogs is easy. The main leash training Milo needed was how to walk in a group, so that had to be together. The only thing I separate them for now (six months post-Milo) is playing with the lure pole. Milo and Romie get too intense.
3. For future possibilities -- might you consider a greyhound mix as a kind of compromise? Milo, dog #3, is a greyhound crossed with probably some sort of huntin' dawg. He is young and fairly high energy yet has some sighthound characteristics.
4. For mental and physical tiring out, never underestimate the power of a good, long chew.