Do you have room for another water heater next to the 85 gallon unit???
In my opinion, only, putting a 50 gallon electric in series with your 85 wouild be a better course of action because it would probably cost less and be far more reliable over time...
also when you dont have company, youi canalways turn it down of off it you set it up right....
the tankless units are nothing but trouble and usually costs quite a lot more than a normal heater.... just my opinion.
read these links about them