Yes, there is! Open the booking and click on the Payments tab. You'll notice a payment form URL that you can send to the guest. That URL opens a secure web form that the guest can use to make a payment at any time using any credit card.
In the scenario you're talking about, you'd want to change the quote so that the original deposit is only the amount that the first person is paying. Then the second person could send their payment.
The payment URL page does take the ENTIRE balance due, so if they are only paying 1/2 of the deposit, you would need them to call you or email you and you manually enter that payment for them.
You can of course always enter a manual payment, using anyone's credit card info, at any time. You can also schedule as many 2nd payments as you want once the card info is on file.
Hope this helps,