This is a little trick I discovered to get infinite coins:

1. Make Mario go in front of Luigi.
2. Choose the hammer for Luigi.
3. After you hit Mario with the hammer, for Luigi, choose the high jump.
4. Use the high jump, but before Luigi lands on Mario's head make Mario jump.
5. This makes a coin pop out from Luigi.
6. You can do this as long as you want but doing this will decrease the health of Luigi, so keep some mushrooms handy.