Maybe. It depends on how cruddy the cruddy cards are, and how decent the decent card is. You could have two $200 cards beating a $400 card or not - it depends on which cards, which game, which resolution...
The general rule of thumb - don't start a build with Crossfire, unless you have tons of cash and you Crossfire high-end cards. Just get the best card you can. Add another later if needed, it will be cheaper too.