>>     Thanks! :)   <<

I didn't know it was so difficult to use spaces here :(

Hi,
Maybe a visual display of the queue and the tills will be helpful :

Q          T = 2         Sum

1    -->   1 | 10 | 6 | 17

2    -->   2 | 4 | 5 | 11

10

4

5

6

At the end:

T1 contains : 1 | 10 | 6      T1sum = 17

T2 contains : 2 | 4 | 5      T2sum = 11

Maybe somthing like that and emphasizing on the part that only empty q will get new customer will help
Maybe integrating the notes after Edit: in the details will help too

I hope that had been helpful