Is it Possible to Combine two digitally signed PDF without loosing the signatures?

Before Corona we got our offer via PDF which was manualy signed be two persons.

We create an invoice for that offer and send it back on the customers portal.

Since you can only send ONE file, we had to combine those 2 PDF.

With a handwritten Signature, no problem.

Now with corona we got ourselfes this PDF with a digital signature.

And we can´t combine those two PDF because it would unvalidify the digital signature.

(ZIP is not a soltion, and sending an unvalid signature ist also not an option)