This is a portion of a PM I sent to DragonLance reguarding his build. This was after I spent about 2 hours testing double weapons with both a Large race (Mino) and Base human which is medium.
You are correct you do not need the 3 dual-wield feats to have extra 2 attacks from double weapons there are automatic. netting you 7 attacks instead of 5. Interestingly enough with the dual-wield feats added in you do get the extra off-hand from imp. 2 weapon fighting for a total number of 8 attacks.
Now it seems that a Large race wielding the weapon does not get extra 2 attacks reguardless of the feats that I could see. The only reason the extra attacks work in Shapstrong is if the Barbians subrace is Medium and not large.
I deleted the test toons, and didn't make any notations, but I could when I get a chance. I also wanted to make sure this is DragonLances thread, after all it is his build, and the missing feats made me question and test. I'll have posting results in the general build forum once I get it finished.
from other threads and so forth the three feats are needed and you must be shapestronged to get the extra attacks...
I would recommend looking at the barbarian build question thread, especially Funky's answer at the very end and perhaps some of my tests of various combinations with all abs shown. With a medium size barbarian, you can have the extra attacks using your large sized double weapons by going into shapestrong form, equipping your weapon, logging out, logging back in, then un-shapestronging. You're now medium size but using a large dire mace + shield for example, and you get the extra attacks with no dual weapon penalties (7 total without HS or dualling feats). Yes you look a little funny as a dwarf with a telephone pole in one hand and shield in the other, but at least you can transition through Illithid doors (some of which the shapestrong model is completely unable to go through).
I'll admit that the large creature size, double weapon, shield, shapestrong, and harper scout combine in confusing ways. Funky's post really seems the best I've seen in making some sense of the double weapon extra attacks.