SQ are usually very punctual so you may be happy to take a risk. They will link the two PNRs for you so you only have to check in once and your luggage will be checked through to final destination. There's a small chance you would have a problem if the first flight is delayed. I have done 3 or 4 journeys where individual legs were booked separately and linked later - e.g. for SYD-HND which came up as no availability but I could book as SYD-SIN and SIN-HND.