Attic Furnace for a garage.
 

Just finished building a garage and we are putting an apartment above it. We were thinking about putting a forced hot air furnace and a tankless water heater in the attic above the apartment. There is really no where else to put it, but I didn't know if something like this was possible. Any advice is appreciated!!

It will be a local code thing, check with your local HVAC people.

It's never a good idea to install any plumbing in an attic or on an outside wall.
In Maine I can see this would be a disaster during a power failure.
You also can not use the same blown air heating system to heat the garage and the apartment if that's what your planing due to carbon dixoide.
Why not just make an equipment room in a closet up stairs or install it in the garage and duct a make up air to the outside?
Is that tankless going to be electric or gas powered?

There is a system exactly for your needs
It is the Unico System
I advise you speak with them on the details of your application
Unico System Inc. - Small Duct High Velocity HVAC The Unico System

The tankless water heater would not be in the attic... Also, the furnace would be responsible for heating the apartment only. The whole system is also on a backup generator.