It is hard to find numbers, but combining phases is definitely how it works for some models, eg Miele PowerFlex uses "booster output of up to 7.4 kW" [0]. This is why you can only use it for one field.

> Stove outlets can handle 12kW

wasn't aware, in that case I was wrong, good for them

[0] https://www.miele.co.th/domestic/1512.htm?&&&info=200004302-...