EV Charging tripping MCB
So, it turns out that one of the cars only charges on single phase. I checked the IEC 62196 Type 2 cable and it only has 5 of the 7 pins present (most importantly it's missing L2 &amp; L3).

My interpretation of this is:
The car is drawing 6.6kW over a single 230V phase, or 29A. Which means that when it's combined with another vehicle, the L1 pole of the 3-phase MCB easily exceeds 32A, causing it to trip.

Please feel free to correct any invalid assumptions...