This trophy is earned through local co-op only and requires an additional controller however an additional player is not required, one player must be playing as Captain America whilst the other must be playing as Human Torch, once both players are on screen in Co-op it will unlock. There is no need to play a level with both the characters however if it doesn't unlock then playing a level might allow it to.
To update on this trophy with the newer features of the PS4 over time. This can be done thanks to SharePlay, all you need to do is host a SharePlay session and invite your friend to come watch, you can then select to give them a controller and choose the option to give them the second controller.
Voila, now you can play a local co-op only game with your friend half way across the world! XD And now you can politely ask them to switch to Human Torch or Captain America, depending of course on who you are. This is how I achieved it as I don't have an additional controller.
I do recommend trying a few Share sessions with a friend if you don't have a local friend in this game because co-op makes this game a real blast!