Haha "Breaking Immersion" seems to be a big topic on this forum now. So much so that people seem to just say it about anything they disagree with.
How would the ability to equip two shields break someone's immersion into the game? If anything, it seems like if you clicked on a second shield and it was like "Nope, you're not allowed to do that. It's impossible to hold two shields in real life.", that would be a little more 'immersion-breaking'.
That being said, I don't care.
Quite possible to wear two shields at once in real life so it would not break immersion, however like in real life it makes very little sense.
I can see one and only one setting where it makes sense, you are attacked but is unable to strike back because you can not or will not kill the attacker because it would require a weapon you don't have, is a NPC you can/ will not hurt or something like a dart trap. Her two shields will increase your armor rating.