Combi-plasma chaos terminators don't come pre-packaged in the plastic boxed set like combi-flamer and combi-melta options do. If we want combi-plasma traitor terminators to use (perhaps in a termicide squad), then the combi-plasma needs to be converted.
In this image, I've hacked apart the twin-linked bolter by chopping off one side of it using a hobby knife and some filing. The plasma side of it was created using a standard plasma pistol with the handle removed. It was the simply glued in to place once the parts were filed down so that they would lie flush next to each other. Although it's not too hard to do, it does take a little time and its probably not a job for beginners given how fiddlesome the bits can become.