Easy way is disconnect the XP drive and then install 98 to the slave drive which the BIOS will treat as the master. Reconnecting the XP drive will return you to XP, but you'll need a boot manager since XP won't recognise 98 installed that way. It would have to be a boot manager that will install on XP though, since installing one to the 98 at the time of installation might not work. Setting that for the 98 to be the active partition might be over-ruled by XP when it was reconnected.
Alternatively the BOOT.INI file could be edited to add the Windows 98.
There is another way if you have a copy of Partition Magic that is suitable for installation on XP. Here's the link:-