In fact not only you get no benefit, but the way they affect your character sheet appears to be bugged.
I had 2 equipped on my Defiant, Shields were like 10,6xx per face iirc. I unequipped one. They went down to 8xxx, I though, "oh, they do stack", then I removed the second one. No change. So I re-equipped ONE of them. Shields back at 10,6xx. So I equipped the second one again, no change. Then when I removed the second one, back to 8,xxx.