Originally Posted by Often1

No. TK should check the bags to ORD. OP will simply collect them at BOS pre-CBP, process through Customs, and then drop them at UA bag drop post-CBP. He should not need to recheck the bags.

If for some odd reason TK will only check the bags to BOS, take them to the UA recheck counter post-CBP. While you will have to wait to have new tags affixed, that won't take log and won't affect payment.

You should be able to see all of this on your UA e-eticket receipts. The receipt should show a segment-by-segment breakdown of the allowance and any waivers and will show $0 as the cost for each of the first two bags.