The two have become friends.
Well, maybe friends is a stretch.
Let's say they have a mutual understanding.
Tuff puts up with Aim following him around,
ramming & head butting him,
and bothering him when he's trying to sleep.
Aim lets Tuff lick the milk off his face after his bottle.
A man of many companions may come to ruin,
but there is a friend who sticks closer than a brother.