In Pandemic: on the brink, can I remove multiple bioterrorist cubes at once for a cured strain?
You need 2 actions, 1 per color. The ability to treat a cured disease is described in the original rules, under the list of actions you can perform:


  Remove 1 disease cube from the city you are in, placing it in the cube 
  supply next to the board. If this disease color has been cured (see 
  Discover a Cure below), remove all cubes of that color from the city you 
  are in.


So it is 1 action to remove all cubes "of that color" if that color is cured. Unless On the Brink rules describe a special exception for bio-terrorist cubes, then this still applies.