Connect two thermostats in parallel
Electrically, there is no problem with it as long as you are using simple thermostats.  Smart 'stats like the Nest might have a problem with it. 

Operationally, a building that is so sensitive to pipe freeze is a nightmare.  Less because of the inevitable tenant squabble (but that too), and more because that also means your house will not last long after a power failure.  You should be able to lose power for a day+ before pipe freeze becomes an issue. 

Legally you are able to oblige your tenant to heat enough to prevent pipe freeze, but 65 is a pretty high bar.  I would not want to try to explain to a judge why my house couldn't handle 50.  The way that would end up playing in court is looking like you're just wasting energy because somebody else is paying for it.  I think the judge would say that's as wrong as when a tenant does it.  The tricky mechanical enforcement would only antagonize and get a harsher judgment. Wouldn't want to be defending that.