Agree with
@arnolddeleon 's short answer.
@acalbear no such thing as an "elementary question" - everyone has different experiences and knowledge. (I am impressed by this forum's willingness to share both experience and knowledge which makes me happy to do the same.)
I have 2 ACs with a 17.64kwh + 3 PW2s system (details in signature) and what I did was to determine all load demands in my household. (I skipped smaller items like lamps and such.) But, for the bigger appliances like the ACs I gathered data sheets.
Basically, looked into amperage and voltage demands at start-up and operational stages to get kilowatts used during operation. Using AC amperage required at startup (LRA; locked-rotor amperage) and with a plan to install soft start devices I determined that for whole house backup I needed a minimum of three power walls.
Without pulling up my sheets I think one of my ACs was 80+A and the other was mid-70sA without the sure starts. Adding them reduced them to an assumed mid-30sA each. That demand is very very short in duration (milliseconds?), but can cause problems since the Powerwall2s are limited to ~7kw of peak power each. (Power = Voltage * Amperage [P=VxI] 16.8kW = 240V * (35A+35A); three powerwalls will handle peak of 21kw demand for a few seconds.)
What I ended up designing for was some amount of existing power draw and both ACs starting up with sure starts at the same moment and that put me at three powerwalls. In hindsight I do wish I opted for four powerwalls in the initial install. That desire is primarily driven by the other design consideration, how much capacity do I want in the batteries. I purchased for need and not want. In the future, as the batteries' retention declines I'll likely need to get a fourth battery to fill in the gap.
Tesla had one sure start on the truck that they installed. Not sure if that was by design or not on their part. When I asked about them installing a second sure start on the second AC they gave me basically a blank stare and not really a good answer. I am still not sure if the installer just did me a solid, didn't have the second sure start in the truck and didn't want to come back, or what. But, having the first sure start installed by them really helped with knowing what the wiring should look like for the second. I ordered a second unit and installed it a few weeks later. The install should be done by an electrician, but handy homeowners somewhat familiar with electricity, schematics, and wiring can get it done with some caution and time.
Back to your specific ask, if the grid is available it is a giant pool from which your house can make demands. During normal, grid available operation the powerwalls will provide your home with capacity, but in the situation that both ACs or other house demands exceed the capabilities of the powerwalls you will concurrently get partial demand satisfaction from the utility. In the app you would see solar production, battery supply, and grid all going into the house if you happened to catch it at the right moment.
In the event of a planned or otherwise grid outage you likely would want to turn off one of your ACs to minimize potential of the powerwalls being overloaded and turning themselves off or under powering (brown out) the house.