guveni 0 Newbie Poster

hi,
I stuck in one problem and I couldn't verify if my solution is the best solution.
The problem is ;
two postman should deliver the items between cities in min time. at first look, it seems to be travelling salesman problem.however, in this problem there is two postman and items can be picked from only one city and delivered to only another city.And postmen can carry only one item at one delivery time.
For ex; we have cities named a,b,c,d,e,f. and we have two cargos to be delivered from b to c and from e to f .And all postmen should come back to their start city.We know the time costs to go from one city to another .
I am thinking to construct a spanning tree and keep time in a matrix array.However, I couldn't be sure how the missions should be shared properly(for the sake of minimum time).

Is there any suggestion for the problem?or Is this the best solution?

thanks in advance;