i would set up the bars at (lets say) 150 pixels apart on the y axis
then upon pressing up or down the bartender would move that distance
Exapmle:
1st bar at 300 y position
1st bar at 450 y position
1st bar at 600 y position
1st bar at 750 y position
If bartender y position > 750 then set y position to 300
If bartender y position < 300 then set y position to 750
(this is how i did it for my zelda inventory navigation)
and depending on what y position he is at set his x postion to the end of the bar.
have invible boxes for all the posible places the customer could stand and have a pick object expression for them to move to it and then have it set to "occupied" or something so no other customer can use it.
customers would also have a invisible box that when the drink overlaps it they would play their drinking animation, etc
hope that helps