Given the wording in Multiweapon Fighting that says
This feat replaces the Two-Weapon Fighting feat for creatures with more than two arms.
I would believe yes, this would qualify as satisfying the prerequisites for Improved Two-Weapon Fighting.
However, strict RAW for Improved Two-Weapon Fighting looks as if it would grant only one of the weapons in the multiple hands to get extra attacks.
In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a –5 penalty.
Note the singular. This would would seem to imply that a multi-limbed creature with Multiweapon Fighting and Improved Two-Weapon Fighting would get a primary serious of attacks, an extra attack with each additional limb, and one additional extra attack at -5 for one additional limb.
A very cursory glance through the Bestiary 1-5s shows no creatures with such a feat combination, but several with special racial abilities involving multiweapon fighting. This supports as RAI interpretation that no, you really cannot stack them this way.
Anyone with other thoughts or examples?
Two recent threads have brought this issue back up to the fore.
Multiweapon Fighting Path
Extra Arms/Extra Attacks
The bottom line is this is a very frequently asked question (especially after introduction of the Kasatha) that has no definitive answer. The other two threads should be able to show how the debate has split off into stances.
This is a frequently reoccurring question. Hopefully it will get addressed soon because until then, there is never going to be a consensus on how this works.
Suffice it to say, there is currently only one acceptable, undeniably true answer when it comes to multiweapon fighting: Talk it over with your GM because there are currently no agreed upon set rules.