To [[Combine]]: [[Activate]] the first item then right click the second (identical) item and select [[Combine]].
You can also combine an entire pile of identical items at once. Activate the second (or later) item in a pile and right click the pile group line (the one with a -/+ sign) and select combine. This will combine the item you have selected with the others, up to a maximum of 9 items from the top of the list down and excluding the one activateactive, into one item with the combined weight and the average quality of the combined items.
Note: Combining will reduce the total monetary value of the final product if they are not all of the same QL. Combining uses a linear average but the prices in game are not linear. Prices are quadratic so a quadratic sum would be more accurate than a linear average.