Split Pay supports one active split at a time today; splitting more than one bill is something we hope to offer in the future. Your one active split can be rent, a mortgage, a car payment, or a student loan.
You can switch which bill you split once your second payment is paid off and you've cancelled your current plan. Cancelling removes that approval, so if you want to switch back later, you'd apply again from the start. See Can I switch which bill I use Split Pay for?