So, in essence, you will need to start over with your machine the get a dual boot setup.
You'll need to remove the existing partition, create a partition for Win98 (use fdisk for that)
Install Win98 into that partition.
Then Install Win2k and when the installation asks you "where", specify the part of the disk that hasn't been used yet and tell it to format to ntfs.
I'm not sure what the smallest partition you could have for Win98 will be but I expect its only a couple of hundred Mb. Unless you're planning to buy the Win98 ntfs reader/writer, you're going to need more space than that to be useful.
Good luck with it,
Cheers,
Tim.