I'm trying to find a two-player co-op browser-based platform game I enjoyed a few years ago. It is not this one, although they have many elements in common.
Like Fire Boy Water Girl (FBWG), one character is harmed by fire/lava and immune to water, and the other one vice versa. Like FBWG, the characters can be moved independently, at the same time.
Here are some other characteristics I recall:
It can be played by a single player – you just have to switch between the characters and take turns to move them
The level designs are simpler than FBWG
The characters can push blocks, or carry blocks on their heads
There are waterfalls / lava falls where you need to use your character to create an "umbrella" for the other character to pass safely underneath
I think it was created by a prolific puzzle game designer (Scandinavian maybe?) and I think I played the game on their personal website