There are two main schools of thought on the matter.
1) The one I follow (which is objectively more complex to realize) is that first thing "boot" and "system" volumes must be separated, in the case of a Windows NT OS, installing it in a volume inside Extended, and in a multiboot scenario a volume must have a same drive letter assigned in each and every OS that accesses it AND that all volumes should be accessible from all OSes installed. The caveat is that more than a few (poorly conceived) programs will insist on installing (or however using files) on C:\, and in some cases they are a nuisance, on the other hand a (very stupid) malware virus hardcoded with C: or blindly wiping the active partition in the MBR will be able to make much less damage.
2) Alternatively, nothing prevents you from having completely separated installs, with the partition(s)/volume(s) of the "other" OS hidden from the "current" one.
This gives a number of advantages, i.e. the install of each OS will be more "standard" so you won't have any issue with programs hardcoded for C:.
Both are valid approaches, what I recommend in the second is to NOT have normally the "other" drive/volume visible, the idea is that before or later one is booted in the "other" OS and makes something destructive on the "wrong" volume because of a same volume having different drive letters under different OS.
Of course, even in this second situation, the "other" drive/volume in a dual boot can be made visible - in case of need - to repair/fix the "other" OS.
Example for #1:
1) small, FAT32 partition primary active drive letter C:
2) NTFS volume (logical volume inside Extended) Vista drive letter D:
3) NTFS volume (logical volume inside Extended) Windows 8.1 drive letter E:
4) NTFS volume (logical volume inside Extended) Common Data/Storage drive letter F:
Example for #2:
1) NTFS volume active[1] primary partition Vista drive letter C: (hidden when 8.1 is booted)
2) NTFS volume active[1] primary partition 8.1 drive letter C: (hidden when Vista is booted)
3) NTFS volume primary partition or logical volume inside Extended Common Data/Storage drive letter D: from BOTH OSes
1st example for a "mixed mode":
1) small, FAT32 partition primary active drive letter C:
2) NTFS volume primary partition or logical volume inside Extended Vista drive letter D: (hidden when 8.1 is booted)
3) NTFS volume primary partition or logical volume inside Extended 8.1 drive letter D: (hidden when Vista is booted)
4) NTFS volume primary partition or logical volume inside Extended Common Data/Storage drive letter E: from BOTH OSes
Of course there are other possible setups, including having the small FAT32 partition have not a drive letter assigned normally, which is what more or less is already happening on UEFI/GPT disks, i.e.:
2nd example for a "mixed mode":
1) small, FAT32 partition primary active no drive letter assigned
2) NTFS volume primary partition or logical volume inside Extended Vista drive letter C: (hidden when 8.1 is booted)
3) NTFS volume primary partition or logical volume inside Extended 8.1 drive letter C: (hidden when Vista is booted)
4) NTFS volume primary partition or logical volume inside Extended Common Data/Storage drive letter D: from BOTH OSes
jaclaz
[1] this approach needs a bootmanager capable of changing the active status of the partition and hide the "other" one at boot time