The problem with having a dual boot phone is that phones typically have limited storage space. If you want to dual boot a PC with Linux and Windows, you can stick in another hard drive to add a terabyte or two more storage. Phones, on the other hand, only have a small amount of space. My phone (a Droid Bionic) has 16GB of storage. It's a bit old, though. Newer phones come with at least 32GB of storage. Of this, some is allocated for the OS.
If you want to have two operating systems on the same phone you have two options:
1) Have the user storage area (for apps, photos, videos, etc) be smaller. Some people will buy your phone because "it runs Windows AND Android" but word will quickly spread about the fact that this means you can't install as many apps or take as many photos as a normal Android only phone (or Windows only phone for that matter).
2) Add more memory to the phone. This will allow you to compensate for the second operating system, but it will also raise the price of the phone. Users will need to decide whether the increased cost is really worth it.
Yes, you can use MicroSD cards to increase the space, but that's an added cost to the user. Telling the user that they just bought this more expensive dual-boot phone and now they need to buy another card to get the same user-storage space as that person who bought the cheaper single-OS phone is a losing proposition.