Board Thread:Theories and Speculation/@comment-30631705-20161130044038/@comment-734582-20161130090024

it seems that when a user obtains a double MS, one sharingan will feature an  offense power and one a utility power. IE Amaterasu is the projection of all-burning fire whereas Kagutsuchi grants control over all-burning flames.

Obito's Kamui is the same: it grants greater space-time control but doesn't allow long-distance projection of the power. Kakashi's can do less but can be used on distant objects.

The trade-off is obvious: having both powers grants more control but having different powers grants more options.