The way I am planning to do this is to make 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 plan 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.)