Question: I hike in an area where packing in all your water is the only option. For longer hikes when it's warmer one bladder won't hold enough. Does anyone make a practical way to connect two together so you have only one feed hose that will draw from two bladders?

Answer: 
I can imagine that there's nothing of the shelf available but this is nothing a little DIY can't fix.  

Go to your local hardware or pet supply store and find a small aquarium hose T or Y fitting that matches the inner diameter of your bladder hose.

Simply cut the hoses of your bladders and attach the two bladders to the T fitting, then attach one of the hoses with a drinking end to the T fitting.  

The hoses should be fine with just a friction fit but you can add two small zipties per connection for extra safety.

PS: This solution requires a willingness to void your warranty
No, pretty sure there isn't one but you do have a couple of options,

* Just bring two bladders and switch to the other when empty.
* There are 6 liter MSR dromedaries that I have used for when you need to pack water in larger amounts.
* There are double hydration bladders with two chambers, but it seems like the idea is to keep water and electrolytes separate, not to double the capacity.

Also, I always bring a Nalgene bottle along as well, in case the bladders get holes in them (this is also best practice from what I have read about hiking in the Grand Canyon).