Simplest way to do what you want - more or less....Create a small fat16 Primary partition - make it 250Meg. max. Install NT there (via usual NT CD boot or the 3 floppies + CD) and apply the updated atapi.sys (from SP4 or later) during installation. Apply the current SP (6a?). Then use Disk Admin tools to configure balance of drive as a single (Extended) partition; format it as ntfs. Afterwhich install NT again and this time point it to install to the newly created/preformatted ntfs Extended partition.
Again apply the updated atapi.sys during setup, and also the SP in use to the completed installation.
You get a dual-boot NT/NT system. The second installation will already have set itself as default OS to boot; leave it that way. The ntfs version is the one you really want...
You can either delete the version in the small Primary fat16 partition - or leave it be. Personally I'd leave it there; useful recovery route/option in the future...
You get almost the whole of the 20Gig as a single NT (ntfs) installation - the 250Gig partition is peanuts, and will not really be missed as useful space. Both installations will boot via the boot/start-up files in the small Primary.
Make the ERD for the main version; maybe one for the Primary installation too. Keep safe/current.
Job done.
Personally I'd use only part of the drive for the main installation; use some of the drive for data only? I.e. Subdivide the Extended partition into two logical-drives. First one is for the NT installation (the boot-partition in M$ NT Speak - where the system files go); the second will be data. Make the boot-partition around 5Gig-10Gig max; it's usually more than enough for NT??? Balance of drive (Extended partition) for data?
Need more input on this one - post back...?