Use different booking forms on page-checkout

Hello, I have a question on how to set different required fields on checkout form.

I use multiple booking in EM Pro version. And there I can choose a form for the checkout page. But I have 2 different forms for the checkout, one for free events and another for chargeable events where the bank account needs to be set up.
I could set a condition in the page-checkout.php so that if booking summary is > 0 use the one with bank account fields else use the other one.
My problem is that the validation is always checked for the form which is set in the multiple booking section in settings.
Right now I did set the form without the additional bank fields as standard. And the other form is loaded when the summary is > 0 but no validation for the required IBAN field was done.
Is there any chance to include different forms in checkout with the correct validation?

It would be really great if someone has an idea how to do this.
Thanks!