OK, let?s put dual-booting a mobile phone into practical terms.
On Monday you felt kind of sassy with your new device so you started off with Android because you had checks from relatives for Christmas you wanted to deposit on your bank?s app.
On Tuesday you drove across state to meet up with family, but knowing coverage was spotty you booted into Windows Phone to use Here Drive+. The offline maps did it for you. They are no more or less accurate than what you find on Android, but you can store more maps (free) on the device compared to Google Maps.
On Wednesday you want to go exploring, but all your friends are on Google+ in a hangout talking about this concert going on later that night. You boot into Android.
Later that day someone sends you a text with an address to the concert, you keep it in your messages because you know you have Here Drive, so getting there won?t be a problem.
Time escapes you, and now you are running late for the show. You switch to Windows for Here Drive, only now the text message with the venue?s address is on your partition of Android. The concert is literally in the middle of nowhere with no standard name, since it is an indy show and word of mouth is all the advertising it has. After some delay, you get the address emailed to yourself which is accessible to either partition.
On Thursday you are driving back, using Here Drive. Unfortunately Here Drive points you in the wrong direction using a street that is no longer available. You remember you have Scout installed but there are no offline maps on Windows Phone. You pull off to a Starbucks with free Wifi and boot into Android. You download Scout and get some offline maps.
On Friday you are looking back through your photos. Android?s access to the Windows partition is restricted unless you root your device. To view all your photos at once you need to plug into a laptop.
Friday night you crack and root your device. You learn the hard way what happens when you accidentally delete a file on your Windows partition you believe to be safe. Unfortunately there is not a utility available (yet) that lets you restore your device?s partition table. It either has to be all Android from here or tempt fate on a warranty replacement (good luck) or paying your deductible for an insurance replacement.
Sunday comes and after a day without a working phone you manage to get everything restored.
People on Android already have the capability of dual-booting their devices ? with different flavors of Android. If I wanted to, I could dual-boot my HTC One and have it run Cyanogenmod and stock HTC Sense. I would just choose which backup I wanted to restore from. It?s not practical though. Neither would a device that has two completely different operating systems entirely.
Dual-booting devices would be for a niche crowd of users that like to tinker. You would have to have devices certified to run two separate operating systems, and carriers would likely spend more than twice the amount of time approving updates to go out over the air. Would you want a device that is ?stuck? on Windows 8.1 and one that is stuck on Android 4.2.2 with little hope beyond a chance update once during its 13 month life-cycle? I?m being generous there, too, because you have to consider a ?better? device comes out every 6 months.
And then what of the relationship of the manufacturer(s) that opt to build dual-booting devices with Microsoft and Google? If either OS runs sluggishly it could have a lasting, negative effect. HTC couldn?t afford to lose more market share with Android. Think Microsoft wants HTC? Nah, they just acquired Nokia and would be better off securing BlackBerry for enterprise solutions.
What about also rans? Huawei? That arguably might happen, but be restricted to Asian markets only.
Samsung wouldn?t do this, because they are going to position their own OS soon.
And Motorola? No way.
Nokia? As if.
I think the best case for a dual-booting device is a tablet rather than a mobile phone. Fewer obstacles to overcome in that regard, and most here would agree with that statement. No carrier control and $600-700 gets you a nice tablet with modern hardware running both operating systems. But, the device would need to compliment both operating systems. That?s just not going to happen though.