First go to the first person model of the same type as the weapon you want wieldable(covenant/human).

Then go to the chuck cloner and copy the bone reflex to you clipboard.

After that go to the first person model of the weapon you want wieldable. For this I will be using the battle rifle.
Go to the chunk cloner and overwrite the bone reflex for the battle rifle.

After that make the battle rifle weapon dual-wieldable.

Then switch the jmad linked to the weapon to the jmad that the bones originally came from.
After that test to see what it looks like.
The first person camera may be a little messed up because the new jmad, you can fix that by editing the first person camera in the weapon tag.
But in the end it should look something like this.

I hope you all make the best of this tutorial.
Thanks you,
DarkShallFall