vibrator placement and movement direction

[immortalx]

Hi everyone

I have a formwork which is 4m long, 50cm wide and has an average height of 10cm. Two external rotating vibrators are mounted underside. As you see in the attached pic, i cant really decide which of the two is the prefered method for mounting them. Its difficult to do it by trial and error because modifications to the mounting plate have to be made.
In addition, should both of the vibrators rotate in the same or opposite directions ?

Thanks in advance