I just found this video. This is another example of companies building things for people recording audio

If you are looking to do interviews on the road this is an interesting choice. Using these tools you can plug two lavalier microphones into your smartphone. In watching the video, it looks easy, but I was bummed that the two microphones share one channel (no separation). That is kind of a deal breaker for me. It would only be an issue if you have someone who is really loud against someone who isn't. I did enjoy the brief review of the rode lav vs. the rode lav+ (the cheaper one sounds awful).

SC6 Dual TRRS Input and Headphone Output for Smartphones ($20- ish)

smartLav+ Lavalier Condenser Microphone for Smartphones ($80-ish)