forces on the first disc= N=mg.....1

frictional force= nmg.......2

torque due to frictional force is found out by

nmgr(2pi rdr)/(pi R²)    a=0 to b=R    =2/3nmgR    (if no silly mistake)

for the first disc.......2/3nmgR=1/2mR2 A..........A=alpha...........1

A=4/3ng/R

for disc1.....................W= 4/3ng/R *t................1)

for disc2.....................W= w - 4/3ng/R*t............2)

we get t=3wR/8ng.......

might be a silly mistake but the concept is very much the same.........

rate if useful......