I have a really old version of magento (1.4.1, don't worry, it's patched )
And I use a combination of multishipping with per item shipping in my old webshop.
In the old version, per item shipping IS per item shipping, even when there are multiple items on the same address.
Unfortunately this doesn't work in version 188.8.131.52. In the new version per item shipping really means per address shipping (or per item shipping just doesn't work?). I've compared the two versions with meld (linux compare program), to find the culprit, but the differences between the two versions are minimal (as far as I can see).
I think it has to do with Mage_Checkout_Model_Type_Multishipping
and maybe the functions:
public function getQuoteShippingAddressesItems()
public function setShippingItemsInformation($info)
but even copying these two functions won't work.
Am I looking in the wrong place?
I really need to have per item shipping, no matter if the address is the same. The carrier I use is the default flatrate, and the config is set to shipping per item instead of per order.
I'm using Owebia Shipping extension for years now, and it works for all kind of configurations.. I suggest you to try it (it is free), and adopt it !