Paypal is really the simplest method.

You don't even need an account.

Select Paypal as the method of payment at checkout, choose "split payment" and enter multiple cards.

It'll even code as "air travel" if you get extra points for that on a card.

And you're going to kick yourself if you get a refund for any reason and it goes back onto a gift card.

Then again, some prefer JFK-KEF-LHR-EWR-LAX-CDJ than JFK-CDJ so I suppose the other methods make sense.