You shouldnt have any problems if stacking DR. But keep in mind that certain mobs can severely drop DR
Same goes for using a shield though, not everything can be blocked, the biggest concern being magic
In NG+(++++...) and Mapworks, how necessary is it to use a shield as opposed to dual-wielding? With Eyes of Grell I never needed to use a shield for all of Elite HC once I farmed them up from Grell.