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
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.