Hi Aaron, check out solution S-067570 in the Solidworks Knowledge base. The way parallel transitions work in PDM, is that only the person(s) who have approved a file can revoke their approval.
For example, you have a parallel transition where both User A and User B need to approve before the file can proceed to the next stage of the workflow. If User A approves the file, then only user A will have the option to revoke their approval. User B can only approve.
The workaround to this (as described) in the KB solution is to have another transition (normal) that goes from the destination state to the source state and uncheck the option "Hide when sibling parallel transition is chosen". This would then allow User B to reject the file even if someone has already approved it.
Hope this helps.
Thanks. I haven't had a chance to try this yet, I got sidetracked, but I'll give it a shot soon.