Dunno if this is what you're asking, but Volkswagen made a
double-clutch transmission:
http://www.histomobile.com/histomob/tech/2/122.htm

"These clutches are connected by concentric shafts to conventional
synchro shift slider-sleeves units, the hollow outer shaft driving per
example the slider-sleeves of the even ratios, (2nd, 4th, 6th ) while
the inner shaft drives the ones of the odd ratios ( 1st, 3rd, 5th ).
Thus the control system can pre-select the next gear while the engine
is in traction. The shift is carried out under load simply by
disengaging a clutch while simultaneously engaging the other."