If your secondary load will always only be that -10dBv high impedance monitor amp then I would use the 1272's unbalanced output. That 1272 unbalanced output is 4dB lower in level than the main out too which is closer to what -10dBv expects.
Without adding another high current buffer, the 1272 can't drive 2 low impedance loads. The output transistor on the 1272 amp has enough current to drive the 200 ohms that's reflected to the LO1166's primary when there's a 600 ohm secondary load, but not much more.
Adding the equivalent of a 1271 would allow a second low z loaded outputl, which is how Neve did it on the big desks.
Edit: I'm assuming we're keeping this all Neve so, alternatively, a 31267 10K:600 on the input of your monitor amp would drop the +4dBu to -10dBv and then you can just strap that across your main 1272 outputs as well as your other 600 ohm load.