It's been years since I read Costello's Bio, but I remember him saying that tremendous animosity grew between them in the 50s...it got to the point where Bud would really slap the hell out of Lou in rehearsals.
For years they were inseparable. Even on breaks on the set they would repair to a card table and play Gin Rummy. They would go to one or the others house after work to play more Rummy, fiercely and for big bucks.
I can't remember what started the bad blood between them.