Question

is inner walls of the guard cells are thicker than the outer walls

Answer 1

yes they are thicker

Answer 2

Inner walls of guard cells are thick and inelastic to help in opening of stomatal pore during transpiration and exchange of gases. When a guard cell enlarges due to entry of water into it,  outer walls of guard cells can expand but inner wall cannot expand. As the outer wall expand, inner walls are pulled apart and stomatal pore get open.

Hope this answer helps,