Yes, you should be able swap the playfields between these 2 and 4 player versions. There should be nothing on the playfield that is any different between the two games.

Find another Royal Flush for sale and sell the Card Whiz. Swap out the new playfield in the Royal Flush. Card Whiz is more sought after more because they made less of them and the game has less score reels to tinker with. A big bonus is more of the artwork shows on the backglass rather than having more score reels. But like Mike said you can drop the new playfield in the Card Whiz but it will still be a two player game.

the actual playfield itself is the same for both