I tried CTP, and no insurer seems to provide split payments.
RMS doesn't accept split payments.
I haven't tried comprehensive insurance (yet).
RMS (Service NSW shop at Botany) allowed split over 3 cards so used 2xVisa1 plus balance on normal credit card.
AAMI (speaking to person on the phone) allowed split payment over many cards for comprehensive insurance.
Called 4 different CTP but none did split payments.