Actually, there is only one hydraulic cylinder.
I'll try to explain how it works (kinda difficult without a drawing).
Each speed brake is mounted to the aircraft by means of two vertical hinges above eachother (of course). The hydraulic cylinder is placed aft of those hinges and just pushes the speed brakes open (away from each other). To prevent asymetry when the speed brakes deploy, a compensator strut is used. At the hinge of the starboard speed brake, the framework of the brake extends a little forward (so when the brake moves outward, this extension moves inward). At this extension (through a pivoting joint) one end of the compensator strut is mounted. The other end of the strut is mounted to the port speed brake, aft of the hinge. The distances between the hinge and the strut mountings (once forward and once aft of the hinge) are exactly the same. So when the starboard speed brake moves outward, it pushes the port speed brake exactly as far outward. Therefore, no matter what the position of the hydraulic cylinder is, both speed brakes are always deployed at the same angle.
Hope it is a bit understandable the way I explained it...