My understanding is that you can basically “combine” magic items, but the second one costs 1.5x as much. So a ring of invisibility at 20,000 gp can be modified to also act like a +1 ring of protection for an additional 3000 gp. Alternatively, a +1 ring of protection at 2000 gp can be modified to also give invisibility for an additional 30,000 gp.
But what about the effects that have specified prices? If you have a plain old masterwork full plate armor, would adding acid resistance cost the listed price of 18,000 gp, or, because armor is a body slot, an inflated 27,000 gp? What if you add that, and then want to add electricity resistance? Since the armor already has an ability, is this considered an additional ability and thus costs 27,000 gp? Would the answer change if it were instead a +2 full plate or a +2 full plate of invulnerability?