Using 3.5mm audio/mic splitter

I have a 2017 10.5" iPad Pro and I am trying to use an audio mic splitter (4 pole TRRS male end, 2 TRRS female ends, each labeled with both a headphone and mic). What I need from the splitter is to record audio and monitor that audio simultaneously in either Garage Band, iMovie, or just the Camera app. My lav and shotgun external mic are 4 pole. My headphones are 3 pole. When I plug both the headphones and the mic into the splitter and record, I hear nothing and there is no recording from the external mic (but there is recording from the internal mic, oddly enough). However, if I use only one side of the splitter, it works fine. I can plug in ONLY the mic in to the splitter, the other end empty, and the audio will record without issue. I can unplug the mic and plug in only the headphones and can hear what was recorded without issue. Using both the mic and headphones together seem to be cancelling each other out. The same happens when I use the splitter along with an adapter on my iPhone Xs Max.

Am I missing something? Thank you in advance for your help.