Best way to drill square tubing (Without drill press)
If I have to drill square tubing with a hand operated electric drill I will use a square and mark a line all the way around the four sides of the tubing at the location along the length where the hole is to be located. Then mark the center locations of the holes on the two faces of the tubing on opposite sides.

Then use a center punch to dimple the center mark for the holes on both sides. Next step is to drill a small diameter pilot hole, one from each side, at the center punch locations.

Now you are ready to drill the finished hole size with the appropriate sized drill bit. The pilot hole helps to guide the larger sized drill bit. Drill each hole separately from its side.

Using careful marking and measurements this technique is the best way to get the two holes aligned nicely across from each other. It removes the worry of the angled drill bit not making the hole in the second side in the right place. It eliminates the possibility of an angled drill bit binding in the first drilled hole and breaking. And finally it gives you the benefit of the pilot hole on the second side which you do not get when trying to drill the second hole from the inside of the tubing.