You do not need boot magic or any third party program to do this.
However this is the way i would do it. Create two partitons using FDISK (98 boot floppy).
Install 98 onto the first partiton and then install XP onto the next. XP will see that there is 98 and will make a few changes when it installs.
Once you have installed XP and restart your computer you will be given the option to boot into 98 or XP.
If you wish to share files between OS's make sure the file system is FAT32 and not NTFS that XP allows.