I don't think the Phantom power has anything to do with the gain setting
If you want to split your mic signal from ONE pair of mics, into two SEPARATE preamps, then you MUST use the splitter AFTER the mics and before the preamps obviously! But you can ONLY run Phantom Power on ONE of the two preamps, NOT both preamps!!!!! Sorry if you already knew that, but Id hate for you to fry some expensive gear by putting Phantom ON with both preamps you'll be testings! If you want to split the signal coming out of the SXM2, then it will DEF color the sound IMO! But yes, leave it at the 0db setting if you're splitting the signal coming out of the sxm2
Just split the signal after the mics, and use phantom on only ONE preamp of the two! That should take care of it and is the exact same way Ive ALWAYS done preamp COMPs
Let us know how it goes & best of luck