Unless you plan to resupply by going out over Kersarge Pass for a pickup in Independence, or unless you will be really moving along, you will need the canister capacity to protect seven to ten days of food per person on the stretch between Muir Trail Ranch and Whitney Portal. This is roughly half of the trail and also the hardest part of the trail. A weekender and an Ursack just won't do it for two unless you are both on a serious weight-loss regimen.
My recommendation? Play the game according to the current rules and carry two, full-size bear canisters. Even then, you will be challenged to get the food in them. Each canister weighs a bit over two pounds; if this is make or break for you, perhaps you should consider another trail. People who spend a lot of time in the Sierra take the rules seriously. You should too.
Trying to sidestep the rules is irresponsible, at best, and IMO foolish as well. The rules are there primarily to protect the bears, not to protect your food for you. If, through your failure to protect your food, you loose it and create a problem bear you are causing problems not only for yourself but for people who will follow you on the trail. Plus, if you get caught without approved food storage, you will get a hefty fine, probably ejected from the trail and, you will have to clean up and carry out any mess a bear might have made with your food.