2 things:
First: The TWF feat cannot be used with a missile weapon. Check out the crossbow description in the PHB. I know, I know, weird place for it, but there it is.
Second: Adamantite has 3 advantages: 1) it works in anti/dead magic. 2) It has uber hardness and HPs. 3) The natural bonus can make enhancing weapons cheaper as shown below.
OK, the +2 bonus from adamantite does not allow you to stack magic enhancements on it, you still need to enhance to +1 before adding special effects. However, this can still save you some cash. If you want to put holy on a weapon, you have to pay 18k gp. At this point, you have a +1 holy weapon. With adamantite, you now have a +2 (not for DR though) holy weapon, and it has cost you 26k. Making a full +2 holy weapon would cost you 32k. The more enhancements you layer on, the more you save since it's a geometric formula but the total is always counted as if it was a +1 weapon. You can effectively break the +10 limit with this.
Kengar, in the case of double weapons, yes you still have to pay double for the adamantite, since you're making 2 separate weapons with it.
Note that it's more effective to make heavy armor out of adamantite since the bonus always works and you still get big savings.
And I still wouldn't allow someone to use rapid shot while making melee attacks.
