No, you can't use the same certificate for HTTPS Inspection and for the UserCheck portal (or any HTTPS portal of the gateway).
The reason can be seen if you look an what is actually used for HTTPS inspection and UserCheck portal:
- The certificate for UserCheck (or other portals) is a standard webserver certificate (server certificate).
- The certificate for HTTPS Inspection is no certificate but a sub CA (certificate authority) issued by an external CA (or an if created by Check Point a root CA). This CA issues server certificates for the access website on-the-fly, which are trusted by the client, because they trust the root CA.