>I wonder if is possible to install both systems in the same machine W2000 & NT 4.0

I'm sure it is possible. If you can't do it with standard installation then you have to use other utility such as Partition Magic to a make a system boot partition.

>
>What about the applications installed, must they be installed twice one per system?

Most apps needs to be registered into Windows registry. So you have to install it for each system. But you can save disk space if you installed in the same directory/partition. And you don't have to worry about different library (DLL) engine cause you are installing under the same Windows system engine (NT based)

HTH