Many people will wear a fanny pack - bum bag, backwards. You can search to find try to find one with one or two bottle holders.
Shifting the weight of two full water bottles from the back (rucksack side pockets) to the front, shifts significant weight. @ x .5 liters equal on liter = about one kilo of weight. I have special stainless steel clips that attach to the necks of two additional .5 liter water bottles and hang from the front of my rucksack harness. This allows me to tote up to 4 x .5 liter bottles. When all four bottles are full, I move 2 kg, or 4.4 pounds, from my back to my front - helping to balance my carry weight.
I did this some years back. I also put anything that I will need during the day in the front pack. This includes: my battery pack for recharging my iPhone, sunglasses, snacks and my field expedient toileting kit (just in case), and my IPhone. I also fold my sun hat and put it in here when I am not wearing it.
My important papers, wallet, passport, credencial, etc., ride in zip lock bags in my cargo pockets. Even if I lose everything on my back, I can still salvage my trip from my pockets and the "belly bag."
The rucksack side pockets now get used for my rolled up rain gear (poncho), hiking umbrella and wind and rain-resistant jacket / parka. The rain gear is accessible without removing my rucksack. My fleece rides in the rear mesh pocket. I rarely need to open my rucksack during the day.
The front bag I currently use also has a horizontal sleeve, atop the main compartment, that can hold a 1.5 liter bottle of water. When I know I will need additional water during the day. e.g. on the Meseta, or when no services are available on a daily segment, I will tote the extra weight - in the front - to (a) offset weight for improved overall balance, and (b) to help others I encounter who may not have planned ahead - it does happen.
Here is but one suggestion for something that would work. My bag was made by a French equipment company that has since stopped making it.
This setup works for me. It may work for you. I use a 38 liter Osprey Kestrel.
Hope this helps.
Tom