To Concentric Twist, Or Not.

When/what are the main reasons for concentric twisting?

Starting to wire a drag bike with Maxx and a CDI. Only using the ignition and data logging of the ecu, no fuel injection so the harness is not that bulky. Shielded wires on speed sensors and coils etc, but is the concentric twisting necessary for larger harness flexability, or is it shielding and noise too?

Great info on the subject here:

https://www.hpacademy.com/technical-articles/the-truth-about-concentric-twisting/

You'll still want to shield certain wiring whether you're concentric twisting or not, and that's covered in that link as well.

In the MaxxECU harness that came with the ECU, the crank sensor and cam sensor wires are 2 core, and a bare copper wire ran alongside then a final insulation around that. The 2 cores are not twisted and the shielding isn't a braid type covering the entire core. Will that still be considered a shielded 2 core wire, or should it be upgraded?

If there is a metallic shield around the core then that's fine. Quite often I find shielded cables where the shield is not a braid like we use in our courses but more like a foil sheet wrapped around the core. This will still be perfectly fine.

Its just a bare copper wire (multi strand) running along the two wires inside the outer insulation.

That sounds more like a Drain wire than a Shield. The drain will assist by acting as a ground path (presuming it is correctly wired into the loom) for external and internal noise, but will not provide any extra shielding. Some AV cables are designed like this as well, with one wire being power, the other signal and the bare wire the ground.