You mean the Dual (ST) block? Just tried it and it works fine.
Double check the phase switch next to the power toggle on each of the sides...if you click that it flips the phase of one side. If I flip the phase of one side then it cancels panned to mono, but when both are the same phase there's no cancellation.
Also may be worth checking with headphones on the device if the output should be in phase but it's canceled, then you may have an issue with the unit. Otherwise maybe a routing issue in the DAW if you have to flip phase of a USB track?