I think that source is incorrect (either that, or you're misinterpreting it). You can put multiple spells (or charges, as the game calls them) in a staff, but only one per cast of Enhant, and they're cast in LIFO (Last In, First Out) order. So, if you put Heal, then Magic Arrow in the staff, when you use it Magic Arrow will be cast first, and you'll need to use it again to cast Heal.

By the way, here's some weird facts about Ultima 6 staves:
* If you place Enchant on a staff, give it to a character with no spellbook, then try to use that staff, you will place a glitch spell charge (with a name like Kal Magic Arrow Magic Arrow) on that staff. If you try to use that charge, the game won't let you, making any charges behind it inaccessible.
* If you cast Animate on a staff, the charges in it will drop to the ground. This can be used to discharge a staff without casting the spells in it (useful if there's a glitch spell in it, or you put the one spell that you should never cast (or at least not save afterword) in it, but it also allows you to see what a charge looks like. You can look at the charge, can walk over it, but can't pick it up. (You can put charges directly in your inventory with the Iolo cheat. They're useless, but I believe the Quality you enter determines what spell the charge is of.)
* I mentioned that one spell you should never cast. If you place that spell on a staff, give the staff to someone other than the Avatar, enter solo mode for that character, then have that character use the staff, the game will lock up in an interesting way. (Just save first, of course, as you'll have to force quit the game afterwords, which on period hardware means a reboot.)