mazzilli zvs both igbt failure
Adding the 2nd capacitor did disturb the oscillation and caused the Mazilli driver to latch up which killed an igbt that caused a huge voltage across the other igbt, that also killed the igbt at the other side because you have to put the new capacitor at the zero voltage crossing instant of time, which is impossible to do at such high frequencies, so at first kill the aux 12v power then add the 2nd capacitor, best decision would be to switch off both of the power sources.