Anyone know if you can attach multiple transactionids on email.send for relatedrecords?

I don't think this is possible. The Help Center shows its type in the relatedRecords object as a number. I also don't think this would work with the email templates, because I think they expect 1 transaction record.

I don't remember top of my head will have to check. But you can retrieve sent email internal id and then use record.attach with type message to attach it to multiple record.