Awesome game concept!
At first i thought "duh, balancing that ball is far too easy",
and evading the spikes in the second game still wasn't hard either. One thing left/right once in a while the other one up/down, no problem.
But when the third game appeard where you had to move around with the WASD keys and touch the squares, things already got tricky. When the ball got out of balance while pressing in the accordant direction you had to move away from the obstacles in the second game again while still not forgetting about the objects in the third one.
Then finally the fourth game appeared, and you really hadn't much time to figure out how things rise and sink by pressing space and evading the obstacles and i died shortly afterwards.
Of course getting a feeling for the game is another thing that you will get used to when playing a while, but this will still keep this game incredibly challenging.
My personal tip is squinting.
You really can't focus on one game and then change to the other - and well, that's just exactly what this game wants from you - playing them simultaneously, not alternatingly.
The game is lacking a pause button, but i guess this would either make cheating far too easy here, therefore you'd be forced to censor the screen then, which would make continue playing very hard.
The game introduction become slightly annoying when playing this game several times, although this also gives you a short break to breath.
As already mentioned in the title, the concept is great, and although the flash is simple that shows that a good idea can sometimes even be more important than careful programming and animation.
I have to praise the autor for this awesome game, well done!