I use the stock lollipop 30A, by Skin1980. Big thanks for his great work!
In the Dual Window, originally you can use only the LG apps. Root and build.prop modding, you can then all installed app in Pad.
Root Explorer, or Build Prop editor. Search this line:
persist.splitwindow.support_all: here originally: false Modding this: true , then reboot. The results in images.
Before you do anything, save, save, save!! If you have bootlop, the factory reset don't help, because the modding occured in system files. Just the rom reflash helped, which re-write the system, with the originaly build.prop
Good luck/modding to you.