Interesting experiment, the problem with the long tube is a sizeable amount of fluid is trapped inside the tube. For a full cum injection I think a short tube could be used and secured tightly on both cocks sealing the urethra's together. Depending on how much semen is expelled it might be possible to have the load up into the others bladder. Then when the second cums he could shoot what is left pressurized in the urethra's that plus his load into the others body.