The send’s on a master strip don’t work like you think they do. All the inserts on the master strip are post master fader (the fader level is applied in the mixer and the inserts come AFTER the mixer). So if the send is set to pre-fader, then the send will have the mix as controlled by the master fader. If the send is set to post fader, the send will have the master fader gain applied TWICE.
What you need to do is make two additional busses. Send the master mix to each of the additional busses. Now use the two new busses as your outputs — and then you’ll have two independent master faders for the mix (don’t adjust the master fader for the full mix unless you want to adjust BOTH outputs).
Alternatively, you could set up sends on the inputs to the mix and make each of those sends post fader. Then the second mix will parrot the first mix.