Download multi windows mod for the LG G Flex to run multiple applications in windows simultaneously. It works on kitkat and upwards on LG G Flex and LG G3. To install it on your android phone, you must root it as it is a system-level modification.
Here are the instructions on how to install it.
1) Download app from here. Open app
2) Give root
3) When root is given, a list of all applications will be shown. Select the desired apps
4) Click Apply
5) Click Reboot to save
After the restart, select the apps you want to run in multi window configuration and you should be good to go.
Keep in mind this APK file modifies your phone’s system files so if you do something wrong, you may screw up your android phone. The mod is said to work perfectly in LG G Pro, G Pro 2, G3 and so on.