Auto-Delegation works for individual sends but not when I use bulk send.

My co-worker and I send documents and need to be able to sign each others' sent items, so we both have auto-delegation set up with each others' email addresses. This allows us to sign off on documents when we use the Send menu. But when we send something through Bulk Send, only the original sender is able to sign and not both of us. Is there a setting that can fix this? Or are you not able to use auto-delegation with Bulk Send? Thank you!