Difficulty feels a bit all over the place, after having it very easy I believe I got introduced to the eyes with about 5 at once. I appreciate a wavering difficulty, but I don't the randomized nature excuses such a big difficult wall. Still, I'm being picky, and once it did get difficult I did start to have fun.
Art and sound I felt were really quite charming.
I did get stuck once I reached the skelly heads with the orbiting shield. I appreciate being able to skip the tutorial though, so a simple prompt or something at the start of every run that tells you about the splitting mechanic might be nice. If you look at Binding Of Isaac, I believe each of Isaacs abilites are drawn onto the background of the first room, completely non intrusive but helpful regardless.
Personally, not a fan of how the character feels to control, it's very floaty. Regarding the splitting mechanic, it's really quite cool, but it feels more suited to a puzzle game. It aside, you've got a nice fast paced shooter, but when you split the pacing comes to a halt. Right now, my strat is to take out everything normally, then switch the split mode to deal with skulls; this approach seemed the most efficient but doesn't at all jell with the fast pace gameplay. Perhaps there exists some mythical strat where I could utilize the splitting and keep up the pace, but doing so would probably involve forcing myself to get worse for a while whilst I try to do both at once and completely fail, and of-course it's not the players responsibility to control the pacing.
Apologies if this review sounds overly harsh, it really is a cool game, I just don't see those two parts uniting. I've done something similar with my game Skyward Descent, where everyone wanted mouse control but I preferred twin-key (WASD move, arrows aim). In the end, even if I as the dev could see something fun with the control scheme as someone who was good at it, but I really should have ditched it and worked with mouse aiming or just found a way to make it more approachable, and I see this game in a similar position.
Whatever potential the splitting has, its easier to just not split until I'm forced to. Perhaps it just needs to feel more like an advantageous thing than a hindrance. Letting the player get used to shooting w/o needing the splitting early on against the bats and stuff seems logical (introduce the basics before adding the twist) but I almost wounder if in this case it's instilling bad habits into the player. Letting them get used to normal gameplay before slapping them on the wrist.
I'm gona shut up now.