It better be a pretty decent insurance policy because plastination is expensive. The thing you may have a big problem with is your local health codes and regulations. Many governments won't allow the possession of a cadaver, no matter how it is preserved, other then at approved facilities. There are still a lot of laws in the world that do nothing but enforce someone else's definition of morality. A dead human body isn't any different than any other dead animal when it comes to sanitation. So why is it you can put a dead cat or pig or cow in your deep freeze for an indefinite period of time, but not your friend?
Logistically, if you are mired in morality inspired laws where you live, your best bet might be to arrange for your plastinated body to interred into a crypt where it would keep until your mate expired. They could then be interred in the same crypt or you could be exhumed for burial with your partner in the same casket.