Pheebie mode and mystery box perk?

So pheebie mode says if you start a round with more than 1 item in your inventory, all items are disabled, and the mystery box perk gives you a random item when you launch nubby. Does anyone know which one takes priority? Want to try it (and take it if i do get a second item) but i'm on a good run right now and don't want to risk screwing it up