Chase&Sheldon, Inseparable

Whenever you cast a Dog spell, create a 1/1
green Cat creature token.
Whenever you cast a Cat spell, create a 1/1
white Dog creature token.
{r}{g}{w}, {t}, Chase & Sheldon Inseparable deals damage to any target equal to the number of
Dogs you control. You gain life equal to the
number of Cats you control.

Though from different backgrounds, they were always
there for each other.