I really like this game, It's cute but it can be improved quite a lot in graphics.
It's nice that you can play single player or with a friend, I played this with my brother, my character was the girl <3
:P

Anyway, here are a few suggestions:
As I said (and it's actually the only negative aspect of this game, at least for me) graphics could be improved. This is just my personal taste, but I don't really like the black bars and white background. However, I understand you might be trying to achieve the simplistic look.
In that case, here's a few things that could make it look better:

-Create some kind of consistency in sizes... I'm not sure what words I should use here, but what I mean is: The levels could be designed in some kind of invisible "block pattern"
Example: all walls could be made of blocks with the perfect size of 20 pixels by 20 pixels
(so a long wall could be 20x80 pixels, as long as you use the basic 20x20 pattern)
... and white spaces would use the same size pattern. of course the white spaces need to be bigger, in that case you would use the size of 40x40 pixels or 80x80 pixels... Basically, you could use these sizes with consistency, to avoid having floors with slightly different thickness, or badly placed... (Think about the kind of size consistency you see in Minecraft. every block, wall, floor and ceiling fits perfectly, nothing is out of place)

Here's a good example: In the first level, the ceiling on top of the male character doesn't really fit with the wall (the ceiling is 1 or 2 pixels above the wall) And the girl character room is slightly larger that the male character room, and there's no need for that... These are just small details, but I see these everywhere and they make the game look unfinished.

Overall, I really like this game. These are just my opinions (I think the game fails in graphics, but other than that, it's a really good game, I'd give it a 5 but I can't ignore the graphics. The only thing I like in these graphics, is the characters, even though they are so simple)

I hope you make new one with more pleasant graphics (Just a suggestion, 8Bit style graphics could work very well this this game, but it's really up to you... Maybe hand-drawn style, or paintings could work very well too)

Good luck ;)