About DW:
Nothing has been clearly discovered about DW as a tanking option compared with twohanders. The main reason people keep mentioning DW is because there's no avoidance twohanders in game yet... and probably will never be. On the other hand you have things like 2.8 tanking weapons and amazingly good quick tanking weapons like
[Last Laugh], which spur conversations about using two of those to boost avoidance, instead of a twohander.
At high levels of gear it might be worth using two _really powerful_ onehanded tanking weapons, but nobody knows for sure yet. Parry-gibs _are_ a big issue.
More on this starting from
DK Tanking Discussion