So I figured it out now, thank you for your reply.
TA does trigger all of the cameras connected to the controller. The KEY_REC_VAL returns the detected position for both cameras and saves them in the register and the following of the argument.
T1 / T2 does either trigger camera 1 OR 2. To do so, you have to set the trigger in the camera setup on the vision controller to "asynchronous trigger" ("camera configuration" -> "extended settings"). Then it is possible to trigger the cameras independently and to set the detected position to different registers.