You gotta really watch it on this one. I have a feeling it's different.
"Two main panels" is common in all-electric homes
It is common for an all electric home, especially in the snowbelt, to need more power than a 200A panel, and be provided with more power, typically 400A. This is NOT a main panel/subpanel situation. Either
- Both panels are main panels with main breakers, with full 400A service wires to both, often with a shutoff switch outside.
- Neither panel is a main panel. The "main panel" is the shutoff/master breaker outside. The neutral-ground bond is made there, and each panel should have neutral and ground separated. They will surely have "main" breakers of 200A or similar rating to protect the panel itself, which cannot handle 400A.
That may explain what you are seeing in panel #1 re: the separation of neutral and ground. (it is also a good practice regardless, not least for "second panel/main outside" conversions like yours!)
Heat pumps cannot work in the coldest of winters, so they require "emergency heat" which is plain resistor based heat. Midwestern homes particularly tend to be in low-density suburbs, and the large lot sizes lead to very large homes in one floor - all of which gives the house a great deal of surface area. As a result, it is common for a large home to have dual 70A emergency heat circuits. That's the reason for such large service. I have a friend with that setup, he'd like 2 on-demand heaters for each end of his long home, and he doesn't have the ampacity in the panel!
A 240V-only panel will seem really weird with the neutrals
Since the two emergency heat circuits by themselves will dominate one panel, it's quite possible this is a 240V-only panel.
A 240V-only panel has no use whatsoever for neutral. It still needs ground.
Due to the design of panels, it can be hard to delete (tear out) the neutral bar. It is much easier to delete the ground bar (or never install it in the first place if it's an accessory bar). So you'd convert the neutral bar to a ground bar, which would necessitate leaving the neutral-ground bar in place obviously.
That may explain what you are seeing.
Unfortunately installers are not much for labeling, and expect "the next guy" (i.e. you) to know what you are looking at.
Generator interlock: tricky
In the first part, I described the unusual configuration found in these 400A homes. These are semi-incompatible with generators - at least it's a lot more work.
Your first option is install a transfer switch where the main shutoff is, and have it switch all 400A. This thing will be a monster and may cost a mint.
Your second option is to install main/gen interlocks in either or both panels. What do you do about the fact that some loads you need to power are in one panel, and other loads are in the other panel?
- Install main/gen interlocks on both panels. There is nothing wrong with splitting a generator's output to have it feed 2+ panels, as long as all the loads you have turned on are within the generator's working capacity.
- Move loads around so they are all in the panel you are feeding with the generator. You will need the blowers and controls to be powered. Those are the smallest loads (~1-2A and ~5-16A, often @120V) in a heat pump system. Next up is the heat pump(s) proper (20-60A @240V always) and the largest is the emergency heat (70-150A @240V always)... don't move those.
Read the documentation/schematics carefully and make sure there is clear separation between the emergency heat and heat-pump motor proper. Typically those are fairly simple circuits controlled via a contactor, with little other interaction with the controls and blowers. But check.
The lunatic approach (if your gen is big enough)
In the snowbelt, weather usually causes power outages. So loss of power often happens during very cold weather, when the heat pump can't work and the very hungry emergency heat is needed.
The real issue is the temperature at the heat pump's condenser. So consider this: Build a "gazebo" around the heat pump, with room for some other stuff. Have insulated "walls" you can install on demand. And then heat the inside of the gazebo to keep it warm enough for the heat pump to work happily. Now you don't need emergency heat. When the cold snap fades, remove the walls and go back to normal.
Of course, the heat pump is actively chilling the gazebo interior, so you will need a lot of heat. Where do you get it? To start with, the generator. A generator are only 1:3 to 1:5 efficient, and heat pumps are 3:1 to 4:1 efficient - it's almost a match made in heaven. Capture most of the generator's running heat inside the gazebo. Be sure to plumb air intake and exhaust outside.
You will need more. Add any sort of fuel heater which can be setup to vent outside, put that on a thermostat set around 50-60 degrees F. Heat pumps are very efficient when pumping heat from a place that's about as warm as their target. Now, effectively you are making heat using fuel - and simply pumping that heat into the house.