Hi,
You definetely can install both on the same disk, but each must go into an individual partition. When you start, first use FDISK to partition your drive as you want. Then, boot from a startup disk and format the partitions.
DO NOT USE A STARTUP DISK CREATED FROM WIN MILLENIUM.
After you format the partitions, go ahead and install WIN98 FIRST, onto the C drive.
Once you've done that, install WIN2000 to the next partition. You will be given the option from within the WIN2000 installation to format the partition you're going to install WIN2000 on using NTFS.
Once you have installed WIN2000, reboot and you will be presented with a menu asking which system you want to start.
The trick is to install the older operating system first.
The partition you installed WIN2000 on will not be visible from your WIN98 installation, but the WIN98 partition will be visible from the WIN2000 installation.
Good Luck