I had the same experience with my German Shepherds. When I brought Scout home Ranger changed overnight from being a real mama's boy to only having eyes for Scout. It did get better with time but in my case Ranger was never the same.
I know it's very selfish but as much as Lucy would probably enjoy a playmate she will not be getting a sister/brother in part for that reason (the other being I travel extensively with her and two would be too much for mein that respect). Pretty awful of me huh?
Looks like it really boils down to the individual dog
Oops - Forgot to add Welcome!