Two beers from 1

Anyone have experience with this idea:

Take a heavy brew like a dopplebock, bottle half normally, then water down the other half.

My wife can't drink the 8.5% beers.  So I'm thinking, couldn't I water down half to a 4% ish beer?  Here's what I'm thinking:

Bottle 3 gals of the dopplebock normally.  Boil 2 gals of water, let it cool appropriately, then add to bottling bucket with the remainder of the dopplebock amking it a normal bock.

Thoughts?  Warnings?

Although I don't have any experience with this. I'm thinking, assuming you're doing a full boil, that if you take three gallons of your wort, cool it and pitch normally and then water down your remaining wort to your desired gravity that you will have better results. Watering down your wort to a specific gravity will give you a better idea of what abv you will end up with. I don't think watering down fermented beer is a good idea, I think you will end up with just watered down beer. Watering down wort to a specific gravity and then fermenting will still give you a full flavored beer and you will also know the abv you end up with.

Mike mcdole on tbn has talked about doing this a lot.  Works for him.

He boils water to pre sterilize it... And carbonates the water in a separate keg.  At bottling time he blends as desired. 

Mike is a many time award winning brewer with numerous medals in the 2nd round of nhc, as well as winning the Sam Adams competition one year. If it works for him...

My wife likes the lighter beers as well.  I've done the same thing.  In the bottling bucket if you add the correct amount of sugar without dilution, then add sugar to plain water at the same concentration you can blend and know every bottle has the correct amount of sugar regardless of dilution.