I think the best way to think of it is that the USSR essentially represented a third side which made alliances with whomever was convenient  Early on the USSR was considered by the Allies to be in the Axis camp if not officially then by action during the Winter War there was consideration by France and the UK to send troops to Finland to aid against Soviet aggression though they also planned on leaving troops stationed in Norway to hinder German access to Swedish iron  It was only after being attacked by Germany that the USSR joined the Allies and even then it was out of necessity basically an enemyofmyenemy thing  As soon as the Germans and Japanese were defeated they went right back to being their own again hence the Cold War  So no I dont think putting them as cobelligerent is to much  In the grander scale of midth century history they were basically just a cobelligerent of the Allies as well