What do you mean by slave? Do you mean adding a second Hard drive to a Win98se PC and installing Win2K on it, whilst still being able to access your Win98se drive and it's data?
Then yes. In fact the process of installing Win2K after Win98 causes an automatic dual-boot setup to occur. You can then choose, every time you boot, which OS to use.
In terms of accessing each drive from the other, remember Win2K will, unless otherwise instructed, install an NTFS data system which cannot be accessed from Win98, although the reverse is not true.
The default installation might also make each drive invisible to the other, but this is easily remedied.
If you go ahead, ALWAYS make a full backup before installing the new OS, just in case.