Do doubles yourself with your scanner at home. It gives you more options, time, and creativity. Do B&W-B&W doubles, Colour-Colour doubles, even B&W-Colour doubles!
Let your imagination do the work, you can do doubles as easy as pie.
First, You can pick any negatives that you have, stack, see the image as the result of two negative that you stack.
Second, if you are satisfied enough, put you negatives on your scanner, let your scanner work, and Voila!
Tips: Use 48-Bit Colour option for more variation of doubles :D