That is a spiffy setup, but if it turns out to be an expensive install you would do fine with leaving the wiring/breaker you have and just adding a second Tesla EVSE to the circuit.
So, what's the point of the 'load sharing' then? The reason you load share is so you can have two devices on the same circuit where each one is (say) 80 Amps max, and the circuit itself has a total maximum load capacity of 80 Amps (80% of the 100 Amps for continuous load). You are only using a TOTAL of 80 Amps across the two devices.
I am not picking a fight, just stating what I know and my opinion, plus answering your questions. As to your question "Why not try NO additional cost before spending?", simply because it has no chance practical chance of being a sufficient solution (hence my ask to you to find one person who drives 50 miles per day and is happy with only 110V charging), and finding out it's not sufficient after delivery can be very inconvenient and that inconvenience may be the reason to pay for a rush job later, which is more expensive. That's all, nothing personal.
If you have both HPWC on either a shared 100A or separate 100A circuits, there should be no code violation but double check with your local electrician. Do tell him that when interconnected (with a separate communication wire) and correctly setup, the two HPWC's will not draw more than 80A combined - if they did each draw 80A simultaneously, it would not be unsafe but may trip your main service breaker, so this it to help alleviate the electrician's concern that you don't have a big enough service to handle the setup.
The purpose of load sharing in this case is to prevent the main service breaker from tripping. For example, if you have a 200A service to your house, and then each HPWC on a separate 100A circuit, they could combined draw 160A, which combined with the rest of the house will likely trip the main breaker. Safety wise you are covered, since the main breaker will protect the distribution panel and the individual breakers their circuits, but it would be very inconvenient to keep having the main breaker trip. Having the HPWC's load share would ensure no more than 80A combined between the two. I don't know the code in all places, hence my suggestion to the OP to check with his electrician, but in most places I lived the breakers in a 200A distribution panel can add up to more than the main breaker since it's unlikely they would all peak at the same time (and if they did, the main 200A breaker would trip). Worst case scenario, the OP can have both HPWC's on the same 100A breaker if that will be needed to stay within the code.
Then having one on 50A and one on 100A circuit would not work. Too bad. I think it could be useful to have a dual HPWC setup where one is 40A and the other 40/80A depending whether the 40A is in use. I wonder whether anyone has reverse-engineered the inter-HPWC communications - IIRC it's a serial connection.
I don't think I understand the question. Typical modern residential service in the USA is 200A, in some places you can upgrade to 400A.
Oh, I see now. The 80 would be on a new 100A circuit, while the OP could leave the old HPWC on the existing 50A circuit - it would save work/money not having to rewire the existing HPWC with bigger wires. The first HPWC would be capable of 80A and the second of 40A, but combined never more then 80A. That seems to not be possible as per @brkaus since both HPWC's share a max current setting, so if you did do that, the 40A HPWC could attempt to draw 80A and blow the 50A breaker. In the OP's situation it would be worse, as the existing HPWC on 50A circuit but set as if it was on 100A, would allow 48A when connected to a 2016 S75D car, so not trip the breaker but over the allowed sustained 40A.
