Let's try for a little condensed clarity.
If a model has a special close combat weapon, it must use it in hth. If it has two or more 'specials', it can choose which one to use with certain penalties. All attacks are resolved using the special weapon, barring other special rules such as the techmarine harness thingies.
Certain weapons, such as the power fist, require a second of the same weapon to gain the bonus attack for weilding two ccw. Others, such as a power weapon, can be combined with a regular ccw for the +1 bonus attack.