This is up to your GM, most likely no bonus

The requirement for the shield bonus is to wield the shield, which you may be unable to do during Shell Defense

Wielding a shield increases your Armor Class by 2.

The word wield is not defined in any core rulebook so we use the plain English definition:

hold and use (a weapon or tool).

Since you likely can't "use" the shield while within the shell you would not gain the AC bonus. Your GM might allow it though (such as holding the shield over the opening in your shell to qualify as using).