Very thoughtfully implemented. There's a whole bunch of expectations this is subverting to good effect. Initial expectation was for something more chaotic, but we're greeted with calming music and colors, along with a very composed interface. I expected an insta-drop button in Tetris, but then struggled to think of a logical place for it, and as the rhythm of the gameplay settled in, I realized it was unnecessary—it feels like it's more about positioning and waiting for both sub-games, more about controlling yourself to shift focus between the two contexts. (I also expected an instant-restart reminder/button onscreen on game over, but as the calm music played on, I felt my annoyance (at the surprise of the snake-crash) fade, giving me a chance settle more fully before starting the next round.)
The slowly-expanding snake-area is clever too -- it seems like snake would be much easier to ignore with a full-sized board to start, and the difference between the two axes (vertical being so short) makes it easier to fail fast in the beginning, teaching you early that mistakes with the snake are much more dangerous than those with Tetris.
Conceptually, this is super-interesting. It takes these two high-focus reflex-based games and slows them down to the point that individually they'd be boring, building a new game that's entirely different not only from the source games but from pretty much all games—the core gameplay loop is about being deliberate with your attention. As a game designer, I love this.
As a player though, I feel a little more compelled by the music, the rhythm, and the colors—offering a kind of calm that I wish I could reach more often while playing. I still like and enjoy the game as a player, but I can't help wanting it to be more relaxing, changed so that specter of instant failure doesn't hover quite so close. I wish the snake made a different tick (maybe something lower, more of a "tock") when it enters an edge-tile, that it projected its direction similar to the way that the tetris tiles project against the blocks below, etc. The increased reaction of tetris to your inputs (changing the projection, playing comparatively-loud swooshing sounds, visually rotating the piece) and overall action (moving the pieces faster) almost feels cruel in the way it pulls attention from the comparatively still and subtle snake.
Of course as a designer, the asymmetry and the choices behind them are quite interesting. But I guess it's fitting that a game about two competing contexts leaves me with two competing opinions.
Oh, also: is it a bug that you can safely move "backwards" in snake before your first growth? It's tactically interesting either way.