Originally Posted by
RWY02KTM
Entirely upto the agent. I have had UA agents who'd not even check-in bags to connecting UA flights on separate tickets, and I've had agents who'd do it on separate PNR/tickets/carriers. If you are flying out of SFO, I think you have a good chance (the agents there, specially in the premier lanes know how to handle external PNRs), if coming from an outstation, good luck.
It is true that in many Asian Airports (SIN, BKK, KUL, HKG etc). the airlines will do a transfer if you give them your bag claim at the transfer desk when you get your boarding pass, but you do need extra time - as they literally go and find the bag from the baggage claim and re-check/re-tag it. But if your baggage allowance differs, then I am not sure how it gets handled. the SIN-MAA segment may not have the same bag allowance as USA-SIN.
I've had fairly good experiences getting my bags tagged between UA & AI on separate PNRs mostly from the UA hubs and/or very large airports. I think you should be ok on this front.
Worst case you can request for baggage transfer in SIN. But as RWY02KTM make sure you're within weight limit in case you need to do the transfer at SIN. AI I think would have a much higher standard weight allowance than UA within the same class of travel anyways .