The proper way to install dual booting is to install the oldest operating system first then install the next to oldest and so on.
Also it would be best that you knew what file system each was using so as to be able to find a compatable one.
W98 uses the FAT32 file system but cannot read the NTFS because it was developed later.
XP can use either FAT32 or NTFS so would be best for FAT32 for dual boot purposes yet the FAT32 is less secure than NTFS.
You can convert the NTFS to FAT32 very easily but be warned that it can cause the loss of data sometimes.
You would also need to know about any other limitations that are involved such as disc size limitations:
The microsoft formatting utillity limits itself to only formating FAT 32 to 32 gigs or less while NTFS limits itself to formatting 132gigs. There are patches for the NTFS in xp sp2 that open this limit up.
The proper proceedure would be to create a 32 gig partiton and install Windows 98 to that partition. Then create another partiton and install windows XP to that one. and it would automatically set up the dual booting.
Now you can still do what you desire by using a third party boot manager like GAG (free to download and use) With GAG you can create the partitions and install the operating systems (up to 9) in any order because the GAG boot manager hides the individual boot files from each operating system...you would still need to format accordingly or if you onstall XP on NTFS then you will not be able to see the files from within W98!