Sony HVL-MT24AM Operating Instructions Manual page 24

6
Select the flash.
Press the A-B button on the back of the macro flash controller to turn on the
twin flash A lamp, twin flash B lamp or both twin flash lamps. (The lit lamps
indicate which flash will go off.)
As viewed from the rear (control panel side), the A flash is connected to the left
side socket and the B flash to the right side socket.
 For the effect of selecting the flash, see from page 51 to 53.
A flash

B flash