37 CFR §42.222

1. (a)Multiple proceedings. Where another matter involving the patent is before the Office, the Board may during the pendency of the post-grant review enter any appropriate order regarding the additional matter including providing for the stay, transfer, consolidation, or termination of any such matter.
2. (b)Request for joinder. Joinder may be requested by a patent owner or petitioner. Any request for joinder must be filed, as a motion under § 42.22, no later than one month after the institution date of any post-grant review for which joinder is requested.