4seats AGV travel in straight line model...

I want to make the model a AGV having 4 seats to travel in straight network line. Attached model is not finished.
And the AGV has 4 fixed seats of two loading and two unloading usage seats.
#4 & #2 seats are for loading to AGV, #3 & #1 seats are for unloading to AGV.

The problem is followed;
1) AGV stop location for unloading & loading is not elaborate.
2) Unloading & Loading at one position should be done at one time but One position is done and the other position is not.
3) Model stops during running.

I need someone's help.

I attached the model.

I solved partially my problem of above issue model. But one problem is not solved and stuck me.

That is "AGV is not unloading a box of two in exchange zone in last step".
I do not know how to control the task executer of the last one item not exchanged.

This line condition and process are followed;
1) items enter to main line like yellow arrow.
2) Long 4 seats AGV loads tow colored items and transfer to 1,2,3 and 4 station (in reallity 10 workstation) and exchange one color item to grey item in one workstation and do exchange the other color item again in other workstation.
3) AGV transfer two grey items to exchange zone and exchange them with two new color items.

Condition is followed;
AGV is only one but it has 4 seats which two is for loading (L1,L2) and tow is for unloading (U1, U2).
AGV must transfer two items loaded to 1,2,3 or 4 station on first call condition.
Two items are loaded and tow items are unloaded in excahnge zone At one time , but in work station one color item is unloaded to workstation and the finished grey item is loaded to AGV and in the other work station is same like the previous.

I want anyone's help.