From the main menu, go to Fight -> Versus -> Tag Match. Change the Fight Settings so the COM Level is 1.
In the match, let the CPU beat you up until both of your characters have Power Blow (their life bars will be flashing). Do a Power Blow and mash H+P+K during the Power Blow. If done correctly, your first character will do a Power Blow, then the second will run in and do a Power Blow.
You can also do this with 2 controllers if you don't want to mess around with the CPU.