You'd have your adventure on the JPN Cart and then you'd have your ENG Cart. You would have to restart/redo your adventure on the English cart. All the decisions you made would need to be done over again. You would have to capture the same Pokemon and do the same quests. Basically, your progess resets back to 0, unless you want to continue using the JPN Cart.
I only want to do things once. The fact that Pokemon carts only have one file that you save on makes it a very personal experience. That cart is my game, my journey. Experiencing it the second time isn't as fun because its still the same game. Nothing is going to change.
Do things differently? There's tons of replay value. There's tons of different ways to play.