Hey Jimmy,
For most trips the 20mm mats will be fine with the underlay, but not without.
It's just with the 40mm you can literally get slammed down on your knee and not feel the floor below so I think it would be excellent idea to avoid impact injuries. the 40mm mats don't feel much different than the 20mm when rolling on them but they really perform when you land on a knee or your side with someone on top of you.
So my choice is 40mm but we couldn't afford all 40mm when we purchased so we did 3/4 20mm and the rest in 40mm.
You could probably do the same as they interlock the same.
Steve