thoughts on split bike paint job

could it work if i tried without the tape because then i would get like a cool merge effect.

Honestly this will turn out horrible unlike you imagined unless you have the steady hand of a brain surgeon.

I would avoid repainting your bike even if you'd mask and prepare it properly (have you thought of how much work it is to spray your bike? Sanding it down, primer/base coat, actual paint coats, clear coat, et cetera).

Even if you're finally done and the result is semi nice, the paint will never be as good and strong as the original, factory paint. Repainting your bike sounds like a cool idea on paper, but without the proper tools and investing a lot of time in it, it will turn out not so great in practice. Just my two cents though.