(it is often possible to receive a higher level set piece from 2 lower level pieces)
This of course assumes that you have extra items belonging to the same set. The best way to nearly guarantee that you get an item from the same set, is to use items from the same set to transmute. This is especially important if you are rolling for a particular set item. For example, for "A Random Unique" transmutation if one uses four uniques varying in quality 19-32 the resultant item will lie between these quality levels, on the other hand if all four are of 23 quality level then the resultant unique will have 23 quality.
It is similar to transmuting in the first game, but features some different recipes an average of the quality of the ingredients one uses. Transmuting in Torchlight II is a process by which the player can combine certain items into new items.