To [[Combine]]: Activate the first item (double click when it's in your inventory), then right click the second (identical) item and select [[Combine]].
You can also combine more than two items at once. For this to work, activate any one item but the last item first in a pile of identical things, then right click the first entry (the one with a -/+ sign) and select combine. This will combine the item you have selected with the first 9 from the top (not counting the one you selected) into one item with the combined weight and the average QL of the combined items.