For my campaign mod, I was wondering how I could make some of my AI hold dual weapons, like PRs and SMGs. I know CMT had the Ranger Elites with dual PRs, but I don't understand how they did it. Can anyone help in this?

I'm probably mistaken but...
You would need to create a new weapon that the model of was two of the models for the regular combo. Then you place the left hand placement on the left gun and the right on the right gun and link them to each other so that they move together and you go about the process the same as you would with a regular weapon then put it in game and test it out.
I might be wrong.

I cant even get CMT Ranger Elites to work, I keep getting some missing tag error

If it has to do something with reading where the tags needs to be placed, I think you need the tags in C:\.....Halo Custom Edition\Tags\CMT\Characters\ and from there place the character bipeds in there, and weapons is everything but the characters thing.