mmm echoing assorted comments above...
It can be done.... '98 in after XP etc..
In your situation - perhaps initially check whether or not the '9x compatablity options within XP will suffice for said games? It may allow all you require?
Perhaps a read of:
http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/appcompat.mspx
may help with that approach?
Otherwise either you use an add-in boot-util - as already mooted by others above,
or:
First (with XP set as Master) copy the XP boot.ini (and maybe the ntldr/ntdetect.com) to a floppy, lock and keep handy... Next set the '98 drive as Master, slave the XP drive to it, install '98 to the fat32 formatted "Master"; then copy the contents of the floppy you made earlier to the c: root of the '98 Master. Then open the copy of the boot.ini (now on '98 c: root) - via notepad/wordpad - and edit the entries there for XP that show rdisk(0) so as to read rdisk(1); save changes and exit. Then start an install of XP (CD or 6 floppies + CD boot - not from within '98) to the '98 drive (as c:\temp\win-xp) - cancel it at first reboot; remove all disks and you get the XP boot-menu - listing '98, XP on its (slaved) drive, and an incomplete installation to the '98 drive.
Boot to either '98 or XP (on slaved drive); locate/delete (on the '98 drive) the c:\tem\win-xp folder; also any other folders/files that start with the $ symbol. Delete any entries in the boot.ini (on the '98 drive/c: root) that refer to the incomplete installation.
(When you boot to XP (via the dual-boot menu intially) perhaps useful to set XP as default OS to boot?)
Verify you still boot OK to XP on the slaved drive; presuming so - empty recyle-bin on '98 drive, defrag the drive to tidy up the scene...
Job done.
Both OS will boot as c:\ etc...; XP path statements will be retained correctly etc. (This is because during boot-up XP refers to the registry for all drive-letter assignments etc; thus path statements are preserved even though drive is no longer Master). XP will assign a letter other than " c " to the '98 drive when it views/accesses it; but this should present no issues. '98 will not see XP drive if it's ntfs.
All shared data must be fat32 (or fat16 - 2Gig max partition).
By installing XP to the '98 drive (as above) you allow XP to create automatically teh boosect.dos - as opposed to manually creating it (as per Doug Knox - which is a viable option...). Unlike in NT/W2K the repair routine in XP does not create a bootsect.dos when it finds a dos based OS presence in the c: root of the Master drive...