If you would like more coins, play through the game's first level (on any difficulty setting). When the level completion chart appears, click on "Change Player" and then click "Continue" to keep going. You can alternate as many times as you like to receive coins each time.
Various Gameplay Hints
You can grind underneath rails as well as on top of them.
Watch out for the white signs telling you that you need to switch direction, and make sure you adjust your drop accordingly.
Just because there's no jump sign, doesn't mean you can't throw in a leap here and there to reach other coins.