The two bills have been coming in the same envelope for years, and have the same due date. (Just got another one in this morning's mail.) I think it's been this way ever since Boston Edison merged with Com Gas to form NStar. I don't remember doing anything special to make this happen.

There's just one return envelope, addressed to a PO box in Dallas TX of all places. If i were still mailing checks, I'd send both of them together with the two payment stubs in this one envelope.