Is it OK to join both negative and both positive cables to power replacement sub?

 

The standard Canton sub is a 6.5" dual voice item, with pretty much smallest magnet I've ever seen on an upgraded speaker.

 

I have bought a replacement Vibe enclosure with a dual voice 6.5" sub fitted, however it only has two terminals (been wired inside using all 4, 2 positive and 2 negative), hence my question about joining the cables together.

Yeah that would be possible  

It either halves or doubles the Ohm of each voice coil

Have a look on this site:

http://www.the12volt.com/caraudio/woofer_configurations.asp?Q=1&I=22#results

If your sub has a higher Ohm feel free to change it in the top right corner.

Remember that it should match the amp