I have no insider knowledge on this issue so I assume these are both entirely legitimate and separate grievances.  The fact that they are being balloted simultaneously would lead me to think the action will take place at the same time.

I'm trying to remember where I read fairly recently one union taking action that coincided with a strike of another union, because they supported the cause even though they were not directly involved.  I have no idea how they achieved that but it must have been within legal limits.