Hi
No, i have only 1 USB device attached: the tuner.(see attached pic)
it is powered by the usb port.
Else there is only the power-cord and a rj45 for the network attached.
the default transfer mode seams to be isochronous.
i changed it based on the documentation here to bulk: linuxtv org /wiki/index.php/Hauppauge_WinTV-dualHD
my observations say: the errors dropped down to 10% (about 3500 from previously 35000)
but there are still lots of errors on the tuners when i have both enabled.
disabling the "second" tuner, eliminates almost all errors