Yeah I assume the DRK MT will always override the OT due to the 20% difference from self use.

if there's two DRK Tanks and you are OTing, you should keep throwing TBN on MT at anything that is strong enough for it to pop minus things like tankbusters because then you'd be wasting MP to bring your DPS up. With BW is pretty difficult to run out of MP.

Though you should conserve TBN for yourself things mechanics like Twinbolt or when you know you'll be taking enough damage so healers have less to do.