This is from memory, pdugg, and a fading memory at that since I have only ever owned firewire and ethernet EyeTV tuners, but many users reported that bus-powered USB tuners were much more prickly than those other tuner types. Pulling power for two bus-powered USB stick tuners on the same Mac, recording two HD shows over that same USB bus never seemed like a good idea, even though there were some who reported no problems doing so. Firewire could handle that with no problem, longtime Mac users know USB is a very different animal. So first thing I'd do is just use one of the cheap USB sticks for awhile and see if you get any zero length recordings. If that goes OK for a while, try plugging the 2nd tuner in and instead of opening any "live tv" windows, just try to record in the background. Start recording one show from the guide, then try to start recording a different one a few minutes later, again, with no live tv windows open.
Obviously, you should be recording to the internal drive and not to another external USB drive.
There was a zero length recording issue many many years ago that had nothing to do with USB tuners, it had to do with EyeTV waking up and recording to external hard drives that didn't wake up in time to be recorded to. But that was ages ago, perhaps in Tiger or Leopard? Hasn't happened to me for a long time, but that is one possible avenue for you to explore, changing the location of your EyeTV archive. But your issue most likely is related to USB power and bandwidth. (I had a EyeTV 250 Plus for a while which was a powered USB tuner and never had any issues with it. But that was the only thing I had over USB at the time.)