Most people may not think too much about having one queue or two parallel queues in their simulation models, but there is actually a rather big difference between the two.
Looking at the screen shot below you can see that these two simulations are identical except for the simulation on the top of the image uses one queue and the simulation at the bottom of the image contains two queues feeding into the work center at the end of the system.
Although this seems to be a minor thing it actually makes a lot of difference.
Work center 4 uses a priority routing in discipline with storage area 4 as the highest priority route.
For this reason all work in storage area 4 will be fed into work center 4 whenever work center 4 has capacity and therefore work in storage area 7 will have to wait until all work in storage area 4 has been completed.
This will have an impact on the time that work items spend in the system and on the percentage of work completed within a certain interval.
If your system does not favor work from one stream over that from another and if the buffer locations are not two physical buffer locations in your system it will probably be best to use the structure in the upper part of the image and just feed all the work into one storage area.
It will deal with all the work in a first in first out order and it avoids you having to make a conscious decision on which queue has the highest priority.