|A '98 drive will transfer OK from system to another... I used to do it often... XP on the other hand will not (easily).|
You "could" use a system of swappable drive caddies/bays; using whichever caddy holds the drive you want to use at any time.
There are a few ways to be able to have both drives in one computer, and boot from whichever.
A basic add-in boot-manager utility would do the job nicely; and they're still around. It would be installed into XP - which would remain the current "Master" drive to '98 "Slaved".
If you were to put XP as Slave to 98 as Master it may not work without a little more work; been so long since I dun it that way (boot-manager utility) can't remember all the details.
If you were to set '98 drive as Master..., you could set the whole system up using the XP boot-loader.
But one must remember that the core requirement for a dos based OS to dual/multi-boot with any member of the NT family (NT/W2K/XP etc...) - "without using an add-in boot-manager utility" - is that the bootable primary "must" be fat16. That is where (once all set up correctly) the XP boot-laoder will finally reside, allowing a choice of either '98 or XP via the XP boot-menu.
Windows up to and including ME was dos-based... If you have that condition (a fat16 primary) on the '98 drive then it's no hardship to make it into a dual-boot scenario. However the chances are your '98 drive is actually fat32 - which would mean a boot-manager utility at least...
Without going into all in depth at this stage... post back with the status of your '98 drive. Is it's primary fat32 or fat16?