When I splice "zip-cord", I cut the two wires offset from each other
about 1.5 or 2 inches.
Then I slip heat shrink tubing over the longer
wire and strip the insulation ~1/2 inch. Then I just overlap the bare
wire ends and solder them together,

..and I bet that it works just fine for you; but to me, not twisting the
wires together in some fashion prior to soldering seems completely
counterintuitive. I don't even think I could make my fingers do itg.
Usually, I simply twist the two ends together and bend them back over
inline
with the conductor, but if I have smaller than optimum shrink wrap
available, that makes a lump which is hard to slide over. In those cases,
I
twist each loose end separately--in line--over the other...sort of a
modified 'linemans splice.'

I do the same two things. The twist is important, since the solder is NOT
supposed to provide the mechanical strength of the connection...it is there
to secure the electrical connection. Or so I've been told.

-John O