Yeah, that's what I meant, but I don't think they work well if you have to solder more then a few parallel wires because then it gets really bulky. Not really good for production work, imho.
Techniques like these are typically for in-situ repair.
The NASA Workmanship's guide linked in a sibling instructs to stagger multiple splices. I think that would help with this problem.
I've tried to make a thinner splice on a four-wire cable, by staggering the joints. But I found it was too difficult to cut, strip, pre-load heat shrink, twist wires, and solder without melting the adjacent heat shrink. I think it's doable if you really need it, but I was just experimenting, so I gave up on it.