Originally Posted by
mottai
These two reservations will have two different United Confirmation numbers. Once the ticket is booked, will United allow these to be converted two one Multi-City reservation so that the luggage is checked all the way in bothways,
No. However, you don't need them to do so. You should arrive early at SFO and present both confirmation numbers. An agent can check your bags through to a separate reservation on a *A carrier, although not all agents know how.
On the way back, you'd be covered by SQ's policies, but AFAIK they're substantially similar.
I wouldn't expect any issues beyond an untrained agent.