<p>I recently bought a Titan Aero hot end which came with a 24 V 30 W heater cartridge from E3D.  I'd like to use this but the cable length is only 1 meter long and I need it to be 2 meters.  The ends terminate with prong connections and there is no polarity to the prongs.  How can I safely extend the leads one meter and then connect to my Duet 3 Mainboard 6HC?</p>
<p>Should the cable terminate with a JST connector instead of the prongs to connect to the Duet board?</p>
<p><a href="https://i.stack.imgur.com/OVTbP.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OVTbP.jpg" alt="heater cartridge" /></a>
<a href="https://i.stack.imgur.com/bucyV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bucyV.jpg" alt="duet" /></a></p>

Heater polarity doesn't matter
The heater cartridges are just large resistors and so polarity is irrelevant. Either can be positive or negative.
You can extend the leads by cutting and splicing in ~20 gauge wires* to a two pin JST connector line you suggest.
*
At 24 volts and 30 watts, you need
wire that is rated to carry at least 1.25 amps
. The US National Electric Code dictates that this should be 20 gauge wire, but their standard is very conservative. Since you don't need to adhere to NEC codes, you could get away with something thinner (ie higher gauge number).