OP had the answer all along. It is specified on the e-ticket receipt as required by DOT rule. One bag.

That happens to be correct. The marketing carrier of his first segment, SFO-FRA with an 18-hour connection at BOS, is UA and UA's allowance is one bag. That allowance will carry through to all segments on the ticket.

Because the connection time is 18 hours at BOS, UA may well require OP to check his bag to BOS and then check it with LH which should check it to BLR. If so, LH may grant OP his 2 bags just because it does. OP should not ask or argue.