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.