I plan to do this by creating an "Input Queue" for each instance of my load balanced services. I will take the input (Either User or what was labeled "Queue" in my question) and put them in a queue. I intend to hash a key value that is in both inputs and use that hash to select the queue the input goes in. This will ensure that one queue (and one service instance) will get both inputs and that they will happen synchronously.
Once I can ensure that, I can wait till I get both messages and then perform the operation for the service (Like Lacy indicated). (Save from any race condition issues.)
