i think it doesn’t work for OP b/c of the TRRS jack on the iPhone which causes some undesired behavior?
should work fine in a TRS context w/ a regular headphone splitter LR LR.
Just throwing out another option, although not inexpensive. Been using this for the same purpose as i already had it for other reasons:
Saramonic SmartMixer Professional Recording Stereo Microphone Rig for iPhone and Android Smartphones
It comes with the cables needed for iOS or Android devices, a mini XLR cable, and is hotshoe mountable for DSLR use. It has two 3.5mm mono inputs for L/R as well as a headphone out for monitoring. It’s powered by a 9V and can supply phantom power to an XLR mic. The level meter and gain control makes it easy to get good levels into whatever device you connect it to. Made really well with aluminum and a nice plastic.
Just got an email earlier today from Roland saying something about a new mixer for mobile phones etc. Didn’t pay much attention but might be of interest?