Suggesting this since in-game help does not mention anything regarding the idea. You should naturally have a higher success rate with both bash and stun if you are using a shield - compared to dual-wielding setup. Probably as much as 20-30% better, so if the final number is, say, 25% success chance, it remains at 25% for dual-wield and gets increased by additional 5% (20% of the 25%) if you're using a shield.