I realize this is a rather vague question, and if the other phone (company phone that has unlimited tethering plus international roaming) is connected to my name there is obviously less benefit (a IMIE catcher is still going to "catch" me). My threat model is not hiding from the government or something extreme like that, rather minimizing my footprint/information that gets out into the ether.
My employer likely does not monitor my phone but I can not say the same for Apple nor the carrier so there is that. I would think the main benefit would be hiding my traffic from apple/carrier and sandboxing off those apps I need which need google play but am I missing something, is that an incorrect assumption? Are there other security (and privacy) benefits to this setup?