How to interrupt active task and implement another task?

For example ,there is a CT.

1.One of operators dispatched by dispatcher load a box from queue,and then travel to forklif-team.
2.forklift unload box from the operator ,then travel to elevator.
3.forklift unload box to elevator, then elevator travel to second floor.

what my problem is that how to load another box immediately from queue ,after one of operators have load the first box.

thank very much for your help in advance!

I think it would be helpful if you could load up a example model.
Because I´m not sure, what´s your problem.

this model is not as same as description i did above. in this model ,i just only change taskexecuters.

Ilivid.