Are the rims you have for the duals already set up for duals? You will need a rim spacer to set them out from the main tires. If the wheels do not have the lugs welded to them for the clamps, the easiest and quickest setup is with the over-the-rim clamps. They just clamp over the outside of the and the J-bolts go through them and hook through the eye-bolts you already have.
The spacer bands will set in the taper at the inside of the bead area and will wedge together when clamps are tightened. Typical width will be 5 to 7 inches.
If the rims have the lugs already welded, you will need to determing what setup to ues with them. Some are snap-on clamps, some are straight bolt on clamps and some are t-rail. The J-bolts come in different lengths to match whatever setup you have... Gene