With a good crimper and a steady hand you can probably make a decent fist of double crimping 18awg wires - but you will be very lucky to get as good a result as your stock cables, which have been built with industry standard tools and without the hindrance of sleeved wiring.
As for 16awg you would probably have to reduce each wire to half its thickness and bind them together with the crimp.
It may be worth looking at the gauges of the doubled wires in the standard cable to see if one of them is one or two gauges thinner than the other. If that is the case your best bet is to buy some thinner wire to preserve the gauge difference and then both crimp and sleeve them together.
But your real problem with double crimping wires at the motherboard connector is the virtual impossibility of having two sleeved wires emerging cleanly from one connector point - and then merging smoothly with the rest of the cable where it loops off the motherboard.
There is a way of fixing sleeve at a midpoint in the wire which would involve destroying a couple of Mini-Fit terminals, and this could help to solve the problem of getting two sleeved wires into one connector point, however, that still leaves the problem of neatly bundling more than 24 wires from a 24 pin connector.
In the end the best decision is to split the wires as close to the motherboard as you can get. If you are happy with using a soldering iron
will show you how to do that.
If not you could try a heatshrinkless knotted joint as follows - there's no need to use Polymorph, you could use hot glue or even a general purpose glue instead as long as it re-insulates the wires and keeps hold of the messenger wire sleeve.
The two key points are to preserve whatever gauge difference you find in the stock cable and keep the joint as close to the motherboard as you can.
