"It wouldn't impact DM one way or the other unless they have separate causes of action against Vivitar and the other defendants."

The fact that the Vivitar case was filed separately suggests to me that either the Vivitar issues are different or there is some advantage to separating the Vivitar case from the others.

If there are different causes of action, is it less likely that approval to combine the trials will be granted?

Is it common to combine trials when the complaints are different?