Best way to reduce bikeweight

[Biggsy]

You are not carrying the weight of two bottles of water for the entire ride if you start off with only one.  You will be carrying less weight before the first refill at least.

You can argue about the worth and practicalities of doing this, but the logic is perfectly sound.

[StanThomas]

Water & the Marmotte - you need a plan that works for you based on where the water stops are.
I will start the Marmotte with 2x750ml bottles but only 500ml in each. They will be pretty near empty at the top of the Glandon. Fill 'em up and make sure you drink on the descent and the run to the Telegraph. I would arrive at the top of the Telegraph with empty bottles *but* it's absolute chaos up there, so top-up one bottle at the stop on the climb. Continue to the feed station at Valloire. There are two water stops on the Galibier and the feed station at the top, so one full bottle is enough leaving Valloire. At the summit, fill them both and, again, make sure you drink on the descent and across to Bourg d'Oisans. There are three water stops on the Alpe d'Huez, so one full bottle is enough here.
Your mileage and tactics may vary.