37 C.F.R. § 42.122 : Patents, Trademarks, and Copyrights — UNITED STATES PATENT AND TRADEMARK OFFICE, DEPARTMENT OF COMMERCE — GENERAL PATENTS — TRIAL PRACTICE BEFORE THE PATENT TRIAL AND APPEAL BOARD — INTER PARTES REVIEW —AFTER INSTITUTION OF INTER PARTES REVIEW — Multiple proceedings and Joinder

(a) Multiple proceedings. Where another matter involving the patent is before the Office, the Board may during the pendency of the inter partes review enter any appropriate order regarding the additional matter including providing for the stay, transfer, consolidation, or termination of any such matter.
(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 inter partes review for which joinder is requested. The time period set forth in §42.101(b) shall not apply when the petition is accompanied by a request for joinder.

37 C.F.R. §42.122