Those who are looking to fly UA into SIN and then transit to elsewhere should check Changi transit rules.
Apparently it is quite limited and airlines need to submit application which routes can be transit to/from. At the moment only a small number of connecting flights operated by SQ group (SQ, Silk Air and Scoot) have received approval.
Transferring from one airlines to a different airlines on two separate PNRs via Changi does not seem to be allowed.
Good luck!