Multiweapon Fighting requires 3 or more hands per creature and legs are not legal (for the prerequisite at least).
Dvati is a very special case I know it can be confusing!
Multiweapon fighting states:
A creature with three or more hands can fight with a weapon in each
hand. It can make one extra attack per round with each weapon.
First of all, a Dvati is 2 creatures with one soul, so unless they grow an extra arm they can't use this feat, plus Dvati share feats so two-weapon fighting is shared between them and gives the same benefits.
Even Races that have more than 2 arms are not able to use their feet to use this feat, and Dvati don't get natural attacks with their legs. You can indeed use unarmed strike with your leg and then use another normal attack with your weapons in hands assuming you have the attacks, but you won't gain extra attacks for free with this method and you still have the normal maluses if you use two-weapon fighting.
Two-Weapon Fighting states:
You can fight with a weapon in each hand. You can make one extra
attack each round with the second weapon.
I guess unarmed strike could count as a weapon?(If the DM allows it) for the purpose of the feat and would be considered as using light weapons but RAW does not seem to allow it.
So assuming your DM allows unarmed strike to be considered valid for two-weapon fighting, You attack with whatever part of your body you want and you can use your second attack with whatever part of your body you want and that's it, no extra attacks, Multiweapon fighting is not for dvatis since they both have their melee rounds to do, so they can two-weapon fighting ''twice''.
Since both get a full round action I can't respond with the 1st interpretation it makes no sense to me sorry if that does not help.
As @KRyan suggests here's some quotes from Dragon Compendium (p.16) that supports my claims:
Twins: A "single" dvati is a pair of creatures that shares a soul.
A dvati character is actually two separate dvati
twins who share a soul. These two creatures move
and act separately but have a number of restrictions
based on their connection.
For example: A 1st-level dvati fighter consists of a pair of twins,
each with the same set of fighter abilities.
You select feats, skills, and other abilities for both twins. Do not
choose them separately. If one twin has Power Attack, both twins gain
access to it. In essence, you build one character who occupies two
bodies, regardless of the class chosen.
For Multi-weapon fighting it requires to have 1 creature (1 body) to have 3 or more arms, a Dvati is one Character, one mind but still 2 creatures (with one pair of arms each and not 4 arms alone) that acts seperately (except for spellcasting since it uses the same mind to do so).