Isaac prayed to the Lord on behalf of his wife, because she was unable to have children; and the Lord answered him, and his wife Rebekah conceived.
Genesis 25:21 (NASB)
A notable contrast between Isaac and Abraham, as it relates to their barren wives, is that Isaac prays for Rebekah. We never saw Abraham pray for Sarah.
But even more, "on behalf of his wife" is literally "in front of his wife."
Isaac prayed for Rebekah. Who knows? The text may be telling us that he prayed with her.