It sounds like they're using a trust to be funded by the sale of their existing homes, to buy the replacement home, with the excess funds to be kept in trust for the maintenance of the home, and available for the support of the survivor. There's nothing mysterious or unfair about this; they're doing this for their mutual benefit, and want to make sure that the funds they committed to it will benefit both of them for their lifetimes.

Interesting because I actually read it as not so hot DIY estate planning ;)

There are two concerns I saw - it didn't say trust and there are issues for non-married couples that need to be specifically addressed. Tenants in common - yikes for the heir of the first deceased.