Seizure

What's better than playing an FPS...playing TWO FPS...AT THE SAME TIME!!! In Seizure you will have to switch between two characters to progress them both through their separate levels, while you are focused on one character the other might be about to be ambushed so don't ignore them.
Two levels, at the same time.
The game doesn't pause one level while you are playing the other. You need to be cautious not to neglect one character while you go into battle with the other. 

Work together from apart
Although both characters are in separate levels they're not completely isolated, you will need to use them to open areas for each other and when they are in the same area on their respective levels they will gain proximity buffs.

Proximity buffs
Proximity buffs will apply to both characters and will include increased damage, increased fire rate, higher defence and  faster recovery.

Adaptive Difficulty
The enemies in the game will get better as you do and if you get stuck on a particular section the difficulty will lower to help you pass, If you don't like adaptive difficulty you can turn the feature off and set the difficulty manually.