Q61

Two connected questions about spellguard rings (from Complete Mage):

a) Do they take up a ring slot on both wearers?

b) If you wanted to add the properties of a spellcaster's spellguard ring to an existing magic ring, what would the price be (the normal price of a pair of rings is 4,000 gp)?

I guess the underlying question here is: is this one magic item, or two?