This is a companion to the other guide, but don't attempt this if you're the joining player in a co-op game. My friend was the host and got it accidentally, so I tried the same and even though I met the requirements, it wouldn't pop for me.
I went back into my own game, bought the next upgrade and it popped straight away, so just something to be careful about, especially as the the upgrades get quite pricey as they go on.