Having said that, OP's lights are built a bit more solid because they look like the 120V parallel-wired style, rather than the series wired tiny lights. I'd be completely comfortable splitting the two conductors down the middle like that. That's exactly what you do to terminate a wire, although doing it in the middle of a cable is trickier because you need to be more accurate. Start the split with a sharp knife at one point, then you should be able to just pull the conductors apart.
For the general idea, I love it and it has me wondering if my gutters are steel. Although part of me wants to permanently hang WS2812 strips and be decorated for every holiday.