You cannot combine your reservations. Technically you must be on the same reservation to apply an RUC or GUC to a companion as per the T&Cs, but I and many others have had success with separate reservations.

What I have never done personally though is upgrade a companion using an RUC when I have already received a Medallion upgrade... I've only ever applied RUCs to both me and the companion at the same time. I believe I recall reading that others have done this - not 100% sure though.

Although now that the upgrade window is open, the chances that RUC upgrade inventory still exists are much smaller... even if they allow this, you may have to waitlist.