You can use just one KH750 with a pair of KH150At least for Genelec, and I assume for Neumann as well, is low DSP latency (about 3-4 ms) so that the monitors still can be used for tracking/dubbing. Some of the other room EQ solutions adds several 10’s of ms, but for just listening that is usually not a problems.
Neumann should offer a larger version of the KH750 so that not two of them is needed to match the KH150.
But if you are going to use all the power of the KH150 then of course you will need two KH750 to match the SPL
I mean, if your room is not huge and you are not going to use as a very far distance and at 85db, probably you wont need two KH750