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Lets say I have 2 windows dual booting on the same harddrive (windws 7), How isolated are they?
I am thinking that there should be up to date antiviruses software on both Windows, and they should be different software so that each one can pick what the other missed.
Well, they aren't isolated at all if they can both see the hard drive.
You are correct that they both should have their own anti-virus solutions though.
The only code in common should be the bootloader. One of the Windows installations will have a bootloader that allows you to toggle which Windows installation you boot to. Some rootkits inject themselves into or below the bootloader, so a single infection from that would probably be present for both operating systems.
Aside from that, the Windows installations should operate independently. An infected Windows install would only directly infect the actively running Windows installation.
But they won't be actually seperated. If they're on the same hard drive they will be able to see and access each other, so one infected install certainly could detect and infect the other Windows install. (And it is probably easier to infect a system that isn't running than one that is.)
Both should be protected.
No, they won't be much isolated.
Each install will be isolated from a normal usage. For instance, a Windows update installed on one Windows, won't be applied to the other.
The main problem, as hinted by Rory Aslop, is that both will be able to see each other partition. So a virus which eg. infected each .exe files on the computer, will infect both installs.
This is often not the case if you had disparate OS. Windows 98 won't be able to read a Windows 8 install, nor will Windows 8 read a Linux filesystem.¹ Thus, it would be much harder that a virus on the former affects the later, although not impossible: remember that there's no physical separation. The virus could include a driver for the other filesystem, it could search the raw disk for the files to infect or, more simple, just wipe the full disk.
As you seem to want them isolated one another, I would make the partition of the OS-not-currently-booted hidden / unassign its drive letter, so a malware would at least need to mount it in order to access there, as opposed to simply infect all mounted drives.
¹ Note that the reverse is often not true.
