From the eligibility T&C:
All members who have one million lifetime flight miles or more earn annual status benefits for themselves, as well as the option to extend their current Premier status to a companion (defined as a spouse, significant other or individual who resides at the same home address, based on MileagePlus profile information) on an annual basis. Companion benefits are available on an annual, not lifetime, basis. Companion benefits expire upon the death of the Million Miler or failure of the companion to meet the eligibility requirements set forth below, including, but not limited to, death, divorce or separation.
and
3. The companion benefit is awarded on an annual basis and is nontransferable.

Italics are mine.
I actually think these slightly contradict and it appears that the status of the companion is entirely up to United to decide.