So far this is probably the most fun variation of "control them both with WASD" that I've played.
Its a cool exploration into the theme. Lots of joining and breaking.
Playing with them separated, you get into this weird state where you're just kind of staring, with your eyes focused on nothing and playing with your peripherals and intuition. Its a really unique experience.
But I preferred playing with the ships joined because of the fun sounds and sights as you upgrade.
The game was really hard for me, perhaps it would be more inviting if the first level was easier. That way the player can feel a sense of accomplishment and is more likely to stay for longer.
I think I would have liked it if my ship kept the upgrades when re-joining them. That way I can play as a super-ship, slip out of trouble, then return without feeling like I lost something. The way it is now, I didn't want to lose my progress (upgrades) so I would just decide to tank a few shots.
I have a fun idea for an interesting tweak: if you're split at the end and your health runs out, it would be cool is only one ship died at a time. That way it would be like you get one last chance.
But yeah, fun arcade game! Nice job!