XDA Senior Member goddamnit has released a mod that allows users to add any application to the multi user interface. It also allows users to add as many apps as they want. This is one of those mods that everyone knew was coming, but that doesn’t make it any less awesome than it’s actually here.
Using it is pretty simple. You flash the mod in recovery and then you should be able to add whatever apps you want to the multi window. Of course, this is an early release, so there are some bugs. First and foremost, you’ll have to flash the mod made specifically for your ROM’s version. Currently, LJ1, LJ3, and LJ7 are supported. There could also be some problems depending on whether your ROM is odexed or deodexed. As per the norm, your mileage may vary.