Interlined from SQ economy to QR business (DPS-SIN-DOH-JFK) with no problem at all. On the inbound we routed luggage to SIN because we had an 11 hour layover. We did not clear immigration or get our luggage; we stayed airside. Next morning we stopped at the transfer desk and got our SQ boarding passes, gave them our luggage tags and they retrieved our bag and routed it on our flight to DPS. We did not have to go through immigration. It was painless. Agent said they will do this at SIN for most airlines (not the LCCs).