Love the style. It gets very challenging, very quickly. Maybe slow down the onslaught of geometry a bit ? :-) Also, I think adding in collectibles, or things to try and grab, would make it more interesting, and introduce some risk vs reward decisions.
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Collectibles would be great. An idea would be a sort of bomb you can only collect on that it spawned, but the explosion affects the other phase. Something weird like that maybe? haha.
Maybe phase shields so that you can clean a phase while being in the other?
There is a lot of room for exploration here, and with something this small I would consider the game really complete.