Hi Susan
In Adagio terminology, Item C is an assembled item. You would need to create a bill of material for Item C, made up of 1 Item A and 1 Item B.
You can do then everything you described, except that in order to invoice an order with Item C on it, you would first have to assemble the required quantity. There is currently no way for the system to automatically assemble as you invoice. It sure would be nice if there were, though.
Some other issues are: a) you would not be able to view on screen availability of Items A and B while taking the order for Item C, and b) sales history reports will be misleading because Item C sales should be included in with Item A and B sales, but they won't be.
Steve