Since I made the pocket for the jeans the need for another one has appeared. I thought this time I'd step up my game and go a bit more pro and decided to buy a twin needle to use with my sewing machine. Here is the video I watched before deciding that my machine (a Singer) could take it:

After watching the video I bought a double needle that fitted the jeans seams perfectly (needles 6mm apart) and was ready to start sewing the new pocket. I wanted to master the technique before doing the stitching on a real garment, so I made a test on a white cloth with red and blue thread colors and white at the bottom. This was the result:

Even though the front looked neat I didn't think the overall end result was strong enough to hold something as hard wearing as jeans pockets, definitely not as strong as two independent parallel seams. Maybe if I was doing some decorative stitching that'd be acceptable, but not for a pocket.

Lesson learnt Double needles on a conventional sewing machine are only useful for decoration, not for strength.