its quite simple really...
https://www.google.at/search?q=viking+weapons&biw=1280&bih=917&source=lnms&tbm=isch&sa=X&ei=RUKMVM-cMoPxUouAgdAF&sqi=2&ved=0CAYQ_AUoAQ#facrc=_&imgdii=_&imgrc=Xt9ZdqVicTfS_M%253A%3BGwfrLxI5Kf_K7M%3Bhttp%253A%252F%252Fdownloads.bbc.co.uk%252Frmhttp%252Fschools%252Fprimaryhistory%252Fimages%252Fvikings%252Fviking_raiders%252Fvk_weapons.jpg%3Bhttp%253A%252F%252Fwww.bbc.co.uk%252Fschools%252Fprimaryhistory%252Fvikings%252Fviking_raiders%252F%3B664%3B465
http://2.bp.blogspot.com/_o_-ZFaQDfwc/TILOkh8_hyI/AAAAAAAAAH0/fx2mvlsjISY/s1600/Museum+006.JPG
just two examples of what has been there at the time. Also some metal pieces that belonged to dane 2h axes, that havent been implemented in the DLC
(not yet at least). Also a simple woodcutter used a 2h axe to cut his trees, that would have been just as useful in a battle, why not use it. But more by outlaws or irregular light troops or raiders or idk who, and not in a regular army.
For me thats just logical, needs no big research, these 2 types of 2h axes should have been added.