I love this idea, it basically merges the Lv. X from the DP days of TCG and the LEGEND cards from HGSS. The Lv. X Pokemon would evolve into a much stronger version of themselves, where they would gain new attacks/Poke Powers/more HP/etc., the LEGEND cards was where you would just have to put two cards together to make a bigger card. Here's a pic of a Break card in full form so you can grasp the idea visually:
See how the Break card covers up the picture and adds a new attack, but the other two attacks remain? That's basically how it works. You get a stronger Pokemon with a new attack, more HP, and a gold picture. The card is in Japanese, but up top it does say "Chesnaught Break retains the attacks, abilities, weaknesses, resistances, and Retreat Cost of it's previous Evolution."