The Multiweapon Fighting feat clearly states that it Replaces two-weapon fighting for creatures with more than two arms. What does that mean?
I've given it some thought, and I can think of only three real possibilities.
1.) For all instances where 'two-weapon fighting' is mentioned, any creature with more than two arms replaces it within the rules with 'multiweapon fighting.' So, all feats, abilities, rules, etc., associated with two weapon fighting apply to multiweapon fighting as though it were the same feat, with any mention of a 'off-hand' referring to All off hand weapons, equally.
2.) The feat simply replaces the two-weapon fighting feat, and thus creatures with more than 2 arms just can't take two-weapon fighting and must instead take multiweapon fighting. By extension, they can not benefit from any of the other two-weapon feats, because they all have two-weapon fighting as a prerequisite, which can not be taken by a creature with more than two arms.
3.) The feat replaces two-weapon fighting as it does in example one, with the exception that all abilities, feats, etc. referring to an 'off-hand' apply to only one off hand, either per turn or in general.