I've had to give up, as it was my last day to return the tuner under retruns policy.

I did more testing, and found that a little while after the GUI reload both the tuners would start playing up. They'd both go to no signal, and a couple of channel changes later the box would freeze.

I checked the dmesg and messages log files from a full reboot, but couldn't see anything in them that looked like it found any USB tuners at all, so I guess there's a resource conflict that can only be resolved once one of the tuners is actually loaded.

I can post them here if they are of use to anyone (PM me). Otherwise I think its best to say 2 USB turbo tuners wont run together.