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: 
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).
The best thing to do would be to simply bring two water bladders and switch the hoses over your shoulder after the first is emptied. Alternately, you could try routing two hoses over your one shoulder. Many bags also provide the option of routing the hose over your left *or* right shoulder, if your bag is like this then you could try routing one hose for each bladder over both shoulders. Trying to rig two bladders together sounds like a wet mess waiting to happen to in my opinion.