It's not really fighting. Certainly, were they human they would both be in bad shape, but they're not. Neither are they really trying to hurt one another.
The one Absinthe does not know is slightly shorter than Sköll, but bulkier and he seems to be taking this marginally more seriously than the other. For his part, Sköll is just having fun, especially when he gets one over on his elder brother.
That is until he catches a certain scent on the wind and stops, stock still. Hati follows suit and the two seem to exchange wordless communication, before the Sun chaser starts toward the tree, his brother staying behind.