Double lay shaft

can anyone tell me what is double lay shaft in gearbox
is it one more lay shaft in gearbox, then more drive can be select?
whhich types of vehcile used it?

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."

thank very much~