I've tried that and i get the message saying that i can't cast a spell that is already in effect.
Here's the trick - Create a spell via "spellmaking" at your local mages' guild that includes the summon effect and another effect. This allows you to summon item A with the original Summon A spell, then use your custom spell, let's call it "Summon A for 5 sec, Summon B for 5 sec" The logic to check for an existing summoned 'A' will not work on the custom spell, so your original summoned A will get kicked to your inventory when you Summon and automatically wear the new 'A' via your custom spell. You now have Item A in your inventory, and are wearing an additional one. What you do with them is your business....