This is just my opinion but one good reason is that some older motherboards are a lot slower to boot with xp than windows 98 I know because I already went through that Another reason is that you can swap motherboards with windows 98 but not with xp
As far as the order. I believe It does matter which one you install first
The order is windows 98 first than xp otherwise windows 98 will overwrite the mbr and you won't be able to boot xp