I've done SFO-NRT-SIN on UA on the outbound. Connecting to SIN-BLR on SQ. That allowed me to get some sleep at SIN. On the return, I took TG BLR-BKK and connected to UA for BKK-NRT-SIN. I remember picking up my boarding passes at a transfer desk at BKK.

Also, check this thread on flying to BLR.