OK, I'm in the upper 20's levels at this point (27 I think) and I've had Dogmeat most of the time. I also have the "Puppies" perk which gave me a new "dogmeat" once the original died (very sad - I took dogmeat from Dogmeat and dropped it on a ledge outside the Vault overlooking the Wasteland).

So now I have Fawkes waiting for me in the museum but when I ask him to join, he says my travel companion would not care for it and that I should come back alone. I assumed he meant Dogmeat. So I sent Dogmeat on his way (back to the vault) and tried again, same deal.
Went back to the vault and said "goodbye" to Dogmeat thinking I'd be setting him free to wander and then get Fawkes to join me. Again, Fawkes tells me the same deal. I have no way of getting Fawkes to join me that I can figure.

BTW - Dogmeat simply just hung out at the Vault even after I said "Goodbye" and he has since joined me once again considering I can't get Fawkes to join.

Any ideas? I was hoping I could have both of them in tow, but I can't even get Fawkes regardless. Is he aware that I svck at shooting and thus he's afraid to come out from the safety of the museum?...