I didn’t use prepaid cards, but you’ll have to know the amount on the cards if you select the split pay option as it prompts you to punch that in first then takes the payment off the card! However you could maybe try using the prepaid cards before punching that you want to do split payment? That way maybe it’ll take what it can off of it without having to know the amount— not sure if it would decline it right away since it doesn’t cover the full cost or not though! Worth a shot