Why a Swedish-Russian alliance would strain during the Thirty Years' War

What if Sweden and Russia allied and entered the Thirty Years' War together?

They could make the war even more destructive, but rivalry over Poland-Lithuania would probably break the alliance and drive another realignment.

A Russian alliance would create a rival threat for Sweden even while the two powers attacked Poland-Lithuania. The expected counterfactual response is a rapid Swedish reversal, joining its former opponent against Russia. An enlarged struggle would also threaten civilians already suffering heavily during the Thirty Years' War. The scale of that devastation makes the prospect of adding another major combatant especially grim. The proposed alliance therefore offers little prospect of a stable northern partnership.