I think that if you hook 2 wires to the brake power wire (make a "Y"), add a diode to each of those new wires, and then attach 1 wire to each blinker circuit it might work.
ie;
Each blinker would receive power when one of the blinkers is selected.
But the blinker power could not cross feed/supply power to the opposite side because of the diode.
(and) Both blinker filaments would receive power when the brake is applied.
{thoughts???}
Ride safe, Ted