Describe a combination of a regular octagon and another regular polygon that will tessellate the plane?

Answer 1

A square.

Octagons meeting at alternate edges create gaps which can be filled by the squares.

Answer 2

A regular octagon can tessellate the plane when combined with regular squares. By placing a square in the center of the octagon and surrounding it with eight octagons, the shapes can be repeated infinitely, filling the plane without gaps or overlaps