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The multi-tuner recommendation from SnapStream (if there is one) is likely to be: use hardware encoding TV cards, like the WinTV PVR-250/350 cards from Hauppauge because they dont put the encoding burden on the CPU. If you think about it, using TV tuner cards that require software encoding for a multi-tuner system would require a more powerful CPU as the number of software encoding cards go up.
Obviously, you can only go to the nth TV tuner that doesnt do hardware encoding even with the most powerful CPU on the market before you encounter massive encoding lagging across the board. In theory, this is not so with hardware encoding cards.
Based on our talks with SnapStream, the shipping version of Beyond TV 3.5 should have a "fail safe" that locks out the use of a second software tuner or any subsequent software tuner other than the one already in use. SnapStream allows the use of one TV tuner that requires software encoding (you can choose which software encoding card even if you have the umptieth software encoding TV card installed) primarily because they are trying to keep the buyer in mind, one who will use an All-in-Wonder card (software encoding TV card plus graphics card) and other hardware encoding cards.
That, however, is not an "officially" supported multi-tuner configuration. In fact, according to SnapStream, any multi-tuner configuration that includes a software encoding TV card will be technically unsupported. (Technically, you could use an All-in-Wonder card with two PVR-250s and not use the All-in-Wonder as a 3rd tuner and still get a supported configuration, since the AIW just functions as a graphics card.)
And while this puts constraints on the cards that you can use for a multi-tuner configuration, SnapStream tells us that they are expanding their list of supported TV tuners. So, if Beyond TV 3.4 isnt "working out for you" due to your current TV card, you might want to hold off till version 3.5. According to SnapStream, for Beyond TV 3.5, any card that follows the Microsoft MCE API should work.