Hello there. I'm looking to install a heat pump tank heater but would like to have a tankless propane that can supplement if needed. Ideally I'd like to be able to place the tankless heater inline and simply have it turn on if needed to get the water to 140F if it isn't already, otherwise to just be passive. Are there any heaters out there that do such a thing?

Yes, you can set up either a parallel or a staged system.

Parallel: Use it for sinks, dishwashers, etc. And use the tankless for showers.

Staged: Use the tank system to bring incoming cold water up to a certain temperature, and use the tankless to bridge the difference on demand.

Either way, it provides you with a buffer of warm water against high demand situations.

I don't know of any equipment build EXPRESSLY for such a setup.

Talk to dealers at Rheem, Bosch, A.O. Smith, and maybe EcoSmart to start out. Also talk to your preferred plumbing contractor. See who he's familiar with, and if he is comfortable putting such a system in.