According to pathfinder, Eidolons may use magic items, but share slots with the summoner. So if you put 2 rings on your eidolon your summoner can use 0 rings since the Eidolon used up both of your ring slots. Slotless magic items are a thing that exists, such as Ioun stones.
This raises the question, is a magical sword and magical shield using the hand slots, or are they slotless items that physically require a hand to use.
If they use a hand slot, then the eidolon using a sword and shield would mean your summoner cannot be armed with their own magic sword and shield. If they are slotless items that physically require a hand to wield, then both the summoner and the eidolon can use magic sword and shield at once. (or some other more optimal weapon).
This question also applies to 4 armed monsters who are often depicted wielding 4 swords at once. Since if those are magical slots then only 2 of the swords may be magical. Similar to how you may wear as many rings as you want, but only 2 rings may be magical.