I have a problem about " priority"

Hi
I have a problem about priority. the situation is as follows: there are two different queues with the same kind of goods.The first queue which gets products from the source and follows the exponential distribution.The other one which has products infinitely. Once it comes a new order, it'll combine with the first queue immediately. If the first queue ran out of its products, the order will combine with the other queue until the first queue gets product.