Yes, there is a Koin cheat. First, you need two profiles, and both must have Koins. Go to Versus, and enter a profile on each controller. Now, there are three tabs on each side of the character select screen: wager, stage select, and handicap. Select the wager tab by pressing the right trigger button on each controller. On the d-pad, left and right change the selected Koin, while up and down increase or decrease the wager. DO NOT WAGER HERE, but it's a good idea to memorize the order of the Koins. After opining the wager screen, select okay or enter or whatever, and select your players. You can either fight the round, or pause and choose player select. Now, have controller 1 hit the right trigger at the character select screen for the wager tab. Press left or right for the koin you want, and up or down for the wager. The profile loaded for player 1 will earn the koins, but player 2 won't lose any.

Oh, and there's also a cheat to rack up Platinum and Onyx koins quickly: go to Konquest, select Drahmin, and get up to the 9th mission, where you're supposed to break the diamond. When the mission loads, pause the game and go back to the main menu. Now select any gameplay mode, and instead of going to the fight you will repeatedly go to Test your Might. If you're good at Test your Might you'll rack up koins real fast.