Useing two yeasts for flocculation purposes will work...if you are lucky. It may take a few tries to get the timing and quantities down. However, when this does work, the resulting beer will have characteristics of both yeasts.
Essentially, if you pitch two yeasts at the start of fermentation, the two kinds of yeast are in competition with eachother. As long as they are both beer yeasts, they can exist nicely. (Note, if you add wine yeast and beer yeast, most wine yeasts will kill off the beer yeast!!!) However, the two kinds of yeast are in competition with each other to take over the carboy. Some yeasts are sprinters and take of quickly, others marathon runners and start slow, but just keep chugging. If you have a sprinter yeast and a marathon yeast, the sprinter will multiply fast than the marathon yeast.... so you when it comes time to active fermenation, instead of a 50/50 yeast population, you may have 80% sprinters and 20% marathon runners.
Depending on which one makes the better beer, and which one provides the flocculation, the 80/20 mixture may be exactly what you want.
Be advised, however, if you are re-pitching from the yeast cake, you are starting with a 80/20 mixture of the yeast, and the same sprinter/marathon issue will cause you to have say...90/10 ratio in the next beer...etc
With some experimentation, I have come up with a WLP830/001 mixture that gives a real nice ALT characteristic. What I do is pitch a vial of WLP830 into the wort at 58F, let the wort rise to 62F over 12 hours and then add a vial of 001. The head start of the lager gives it enough population to compete with the 001, the 001 gives a nice ale quality to the beer, and the lager yeast, finishes the beer over the next 8 weeks lagering very nicely. What I am after is the finishing properties of the yeast instead of the flocculation, but I expect the same factors apply.
Enjoy the fun, and I hope you make a ton of great beers while perfecting your technique!