this emulator is the best by far out there for dreamcast games. Any chance we can see twinstick as a device option as mouse and standard controller are already options and my understanding is that the twinstick is a hybrid of the two?
TwinwStick wasn't special device class, it is variant of Controller device but with few more additional buttons.
go to Dreamcast Joy (1) configuration, assign there "Digital 2" and "C","D","Z" buttons (make sure "Analog 2" is NOT assigned) - and you'll get such controller recognized as Twinstick.
note: if its not clear - that "fancy" TwinStick thing is just cheap dual digital stick + few more digital buttons. nothing really expensive analogue.
note2: many of games not compatible with such devices and will hang, you'll be needed to unassign all "Extended buttons" to make them work again.