Unable to book to SIN, out of SYD as one itin?

Hi all,

I'm trying to book a flight from the US to SIN, but then will have to make my way down to SYD (probably on SQ, but that's not bookable on UA's site anyway), then back to the states from SYD.

The web site will not allow me to do this as a multi destination trip (out to SIN, back from SYD). Is this correct or a bug? I realize I'm going to one continent and back from another.

Also, is it safe to assume that if I were to get it on one record that it would be treated as two one-way tickets anyway, killing any benefit of booking together? Is there any other reason I'd want them paired up?

Thanks, and sorry if this is a silly question.