Collect extra personal data during OR after checkout

Hi there!

We sell holidays via our Hikashop webshop (still under construction).

A holiday is in fact a Hikashop product, the product price is the per person price.
Visitors can book a holiday (buy a product) and they can choose an amount of 1 or 2 of the same product. This way he can book for 1 or 2 persons.

What we need:
A.) - If someone buys a product (eg a holiday for 1 person) and starts the checkout process, we need to collect some personal data from that person. Preferrably we would like to present a form somewhere during the checkout process, or directly after the payment.

B.) - If someone buys 2 instances of the same product (2 persons on the same holiday) then we need to collect the data of 2 persons.

Note: It's very important that the collected data is connected to the order of course.

We were thinking of somehow linking to a RSForms Pro form, and pass the ordernumber to a form field?
Maybe use 2 forms: a single person form and a double person form?

But any other workable solution would be most welcome.

Would really appreciate if you can share your thoughts on this.

Thanks!