Depending on additional loads on your garage subpannel (electric dryer?, supply to bathroom sockets or AC?), you may want to consider installing a second 40A double circuit breaker, run the corresponding #8 wires to a second 32A EVSE, located conveniently closer to your second car, and you can charge both cars at the same time. All depends on the frequency you are charging, and whether you'd like to charge both cars during the same night and if placement of your new EVSE matters.
If you decide to add a second 40A circuit breaker, I would check the gauge of the supply wires going from your main panel to your subpannel, and confirm the two lives actually support 100A at 240V.
Make sure you get the wiring gauge right for the ampacity required by the new load, whether by adding a second 40A circuit breaker (requires at least #8) or turning your existing oultet into 50A (here you need at least #6 gauge to hook up a new 40A EVSE).
I don't know what other loads you have on your subpannel, but running 2 50A circuits from a 100A subpannel for 2 40A EVSEs could be overkill. Maybe adding a single new 50A circuit for one extra EVSE is a good move? Technology will evolve, and in a few years everybody may just want at least 40A L2 chargers which require a 50A circuit breaker and the associated wiring gauge.
Select the correct wiring gauges, circuit breakers, and turn off electricity before proceeding with any electrical modifications.
