On separate PNR's, I probably wouldn't chance it unless you are a member of Global Entry and can blitz past any lines at US immigration and Customs. That said, I flew this on May 10 and we arrived in LAX at 19:30 (yes that early). My connection then was to a CO flight leaving from the UA/CO terminal...after midnight. Not sure how long it will take you to get through security for UA though--assuming you are already checked in for flight. I'll be doing the BKK-LAX run again in about 3 weeks and am toying with trying to do the same thing you are rather than wait for the CO again. Or maybe I'll play it safe again.