I used to do that with the flat top fountain pens to make "extended length" pens. A word of caution, where the tubes meet up is VERY fragile. I had a customer drop a pen on carpet and it broke. At first I thought he had to be joking. But when it arrived I realized what had happened. Where the two tubes meet there is a natural weakness. Then pen broke cleanly right at that point. I put some glue on the inside hoping to create a bond between the two pieces of tubing, but I'm not sure how it turned out. I haven't heard back from him, so I'm hoping that that means it worked. But you might want to consider that potential problem before selling one. I still don't have a good solution.