A second Button to validate form

this is a feature i never seen anywhere, i would like to be able to let the customers choose between to modes of payments:
– the first one is the classic mode already available by default, clic on the button send user to paiment and send form
– the second choice would be to send the user to payment page with another option, allowing him/her to pay in 3 times, then send the formthe same way as the first button does.

if you need a reference to do this button i know that the plugin “Paypal Subscription” is proposing this kind of payment.

The page I need help with: [log in to see the link]