I`ve done them twice,once from Glen Feshie in February and once from Linn of Dee in April.
From Linn of Dee George and I cycled in and dumped the bikes across the river from Geldie Lodge.Crossing the river means you will probably get wet feet unless you are lucky
From there we headed out to the Corbett of Beinn Bhreac and then came back over the two Munros.This was a day trip from Glasgow and we didn`t get back until near midnight.
If you are only doing the two Munros then it`s not as long a day as it looks.The cycling on the way in is at an easy gradient on a good track and quite quick on the way out.The hills are easy too.I`d guess you would get them in around about an eight hour trip.