Totally possible.

Edit: Thinking about this some more...it's actually virtually ONLY possible to do this with hydraulic rim brakes rather than hydraulic road discs, for the main reason that hydro rim calipers typically are run on a "closed" system (no reservoir), while for thermal expansion and pad wear reasons the disc are run on an "open" system. Since the reservoir is usually in the lever, if you have 2 levers attached to an open system, you'll just transfer fluid between the 2 lever reservoirs when one or the other is activated.

The one "gotcha" is that SRAM, in their infinite wisdom, decided to use the levers with reservoirs to activate their hyrdro rim brakes :-/ That said, the Magura TT rim brakes should work fine with a secondary lever "Tee'd" into the line...just don't activate both levers at the same time, or you might crush your rim ;-)

Which brings up another aside...do either Shimano or SRAM even offer a hydro TT brake lever at the moment??

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