Shielding effect- specifically defined?
Generally, inner electrons shield outer electrons. In order to shield you have to be closer to the nucleus, but it's harder for an electron in a higher energy orbital to be closer to the nucleus than an electron in a lower energy orbital. Therefore the shielding effect from inner on outer is a lot more pronounced than outer on inner.

EDIT (after better understanding of OP's question):
One thing to remember is that the inner electron can be on the same side of the nucleus as the outer electron, or it can be on the other side of the nucleus as the outer electron. In one case, the inner electron is being pushed towards the nucleus but the other case results in the inner electron being pushed away from the nucleus. A hand-wavy application of Gauss' Theorem suggests that these effects might cancel out over time. So, the net effect is that the outer electrons have minimal impact on the inner electrons. This argument fails if the outer electrons have greater nuclear penetration, but that doesn't happen by definition.