To the best of my knowledge the Post Office doesn't combine or consolidate. When my wife and I mailed our renewals we'd put each application in a separate large envelope but left them unsealed until we got to the PO. Now maybe the clerk who waited on us was thinking more about revenue than rules, etc., but we asked if we could put the two applications in their separate envelopes in the same larger Express Mail envelope. I honestly don't remember exactly the reason why but based on either his directions or recommendation we mailed them separately. Were they combined somewhere along the line? Maybe but it was transparent to us. We did pay for the tracking service and as I remember they were logged in as received on the same day but a couple of hours apart.

Could also be they handle the mailing of DS-11s totally different than DS-82s since the first requires more work and interaction by the PO. I'm also not sure it would make much difference if mailings were or are consolidated and at what level. Barring peak application periods, and assuming no errors, the typical renewal seems to take around three weeks so mailing time doesn't seem to be a big issue. Might be more about a couple like us who could have consolidated our mailing and saved a few dollars than actually getting our applications processed quicker. Heck I spend way more money on Starbucks coffee in a year than I do in a lifetime of mailing passport applications/renewals.