How to install one SSL Certificate for 2 sites?

Question

Digital certificates

In general, I'm talking. :) There are 2 tourism sites, the Lithuanian Bank should provide us with its payment plugin for wordpress, but it requires an ssl private key, I gave it to them, but they say:
Translation:
"You sent 2 certificates. 2 certificates mean that these are two different sites (m.site.com and site.com) will need two different iBPAY operations.Do you use the same private key in your sites m.site.com and site.com or are they different? same key, then send a certificate request, from which the bank will generate a certificate that will suit both sites and then one IBPAY transaction can be used. Such a certificate must have a set expiration date."
Only that is, at the end they write that one private key should be used by two sites, and how to do this, I just don’t even know what IBPAY is, Google doesn’t really give anything ...

Answer 1

The easiest way is to issue a Wilcard certificate (*.site.com). Or, all operations can be carried out on the main site, and for mobile payments, create a separate page with a light design.