For the joining of multiple layers of heavy leather, as with this axe sheath, I like my stitching awl. It's a heavy duty little tool!
The waxed thread feeds from a center-mounted spool through a groove in the needle.
It's a push-pull operation; push the needle through a prepunched hole, pull back to create a loop, then pass thread through the loop on the other side of the project and pull the needle out. This creates a strong, tight and durable lock-stitch.
When I put something together, I intend for it to stay together!
Though there are certainly machines that could handle the thickness, the precision and control of hand-stitching creates a finished result that I consider superior.
For lightweight leather, I prefer the two-needle method. One piece of the same heavy waxed thread i use in the awl with needles at both ends. The needles are passed back and forth through the same holes, making fast work of any job. Pulling each stitch ultra tight is key!