What kind of car do you have, 700 with rear ditributor?
I would do the best job possible splicing by soldering and using heat shrink insulation. Maybe join the shield by wrapping with a length of bare wire then insulate the hole thing with a large piece of heat shrink. A lot easier than trying to reconnect new wiring down at the ECU.

It is not co-axial, bi-axial or any-axial, the braded shield is simply to reduce interference as the timing signal goes to the ECU. Unlike coax a flaw in the cable such a splice will have no appreciable effect, you just need a good electrical connection.
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David Hunter