Switching drives is done either by clicking F8 for choice of setup, or by changing the boot order via the bios.
If not able to switch drives it was my intention to disconnect one or the other in order to retain W8.1 as well as being able to continue as a Windows Insider
Being like yourself wary about mixing O/S,s, I chose not to use EasyBCD.....Reason see latest post from GABE.
DENCAL, just a note, i use to run my dual boot on separate ssd's as well, and while it is easily done, i found that the dual booting system runs better when both boot partitions are on the same ssd (of course if you have room). but i never had to go through all of that to switch drives. you can easily set it up to give you a quick choice while booting. just make sure win 10 is the last partition loaded. and if you don't already use it, macrium works very well for imaging and restoring partitions. cheers.
However, 10147 is an fbl_impressive branch, so it should update, unless there happens to be a bug preventing it.
Dual BOOT on same disk, different partitions, only if you must because an error or change on first partition would ruin both OSes. On separate disks, you can always have at least one OS working and better chance to repair or salvage either one.
backup images. never had an entire disk screw up. only a partition, and never has it affected the other os. just fyi. on different ssd's i regularly had trouble with the system either losing one of them, or corrupting the start up and not showing it when i booted. but then i read several articles saying that it was a better idea to put both os's on the same disk. this seemed strange to me, because i was always told not to, for the same reasons you listed. but i tried it anyway, with bu images in hand and it has worked to perfection for quite a while now. the confidence comes from having several backup images, and the experience i have had with it.
Stevae You are missing the point. dual booting after updating W8.1 to W10 when finally released and being an Insider will not be possible according to Gabe. See Attached Thumbnail in post #372. for confirmation.