Cutting the brass tubing for box hinges is best done with a saw.
The manual version is to use a fine toothed saw - similar to those used by jewelers – with very light pressure.
The power solution is to use a Dremel or similar hobby motor with a cutting wheel attached. This cuts through the tube cleanly and quickly.
In both cases, a fine rat's tail file can clean up any burrs within the tubing.
If you use the internal tubing you can cut both at the same time. However, it is quick and easy to use the appropriate sized copper wire to insert and then bend down onto the corner of the box. This gives greater flexibility and avoids waste.