To assemble two copper tubes of the same diameter without using a fitting, you can form a sleeve of a few millimeters on one of the pipes. This is to enlarge the inside diameter of the tube to make it reach the value of the outside diameter. You will be able to fit another tube of the same diameter and ensure the connection by a solder with tin or copper.
After annealing the end of the tube, thread the mandrel to the correct diameter and hammer to deform the tube.
You can also use a socket pliers. The sleeve once made, the two pipes can fit.