AFAIK at the moment the only USB T2 tuner that in the past was successfully tested is the Hauppauge nanoStick PCTV 290e, but it's now discontinued by the manufacturer and replaced by the new 292e model. At present the 292e driver is still under implementation and not yet supported.
If you have an empty slot, IMHO the best choice is to buy the Duo2's DVB-T2/T/C internal tuner.