Shield Master feat, second bullet:
If you aren't incapacitated, you can add your shield's AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you.
PHB pg.170
Would wielding two shields apply a +4 bonus to the Dexterity saving throw detailed in the feat?
Best Answer
No.
The relevant rule in the armor section states:
This means that only one shield serves to give you any benefit (including the ones from the Shield Master feat).
This is further backed up from this Sage Advice page from the lead designer, Jeremy Crawford (thanks @V2Blast in the comments)