PRD wrote:You can also make a regular melee attack (or off-hand attack) with the spikes, and they count as a light weapon in this case. (You can't also make an attack with armor spikes if you have already made an attack with another off-hand weapon, and vice versa.)
The only real caveat to TWF is the 'off hand' does 0.5 Str bonus, and, of course, the lower attack penalties if the weapon is light.
Handed-ness is really weird and not defined beyond 'full strength bonus' or 'half strength bonus.' You can't generally make extra attacks regardless of how many extra arms you have*, but all extra arms I can find reference to (like the alchemist's) are treated as normal arms. If you've got 4 arms and a tentacle, and they all hold short swords and you want to TWF, pick two limbs- one is primary, and one is 'off' and it doesn't seem to matter which is which. The rest do nothing (but you can have a wider variety of weapons armed at once, plus a free hand for spellcasting and a shield, and that can be a big deal.
*barring however the monster feat multiweapon fighting works, but that is a weird-ass can of worms, involving (probably) DM approval and easier requirements.