Sure you can close one tube off if you had bothered to read this thread you would have seen that has been done ,but at a cost of engine performance .Check it both ways both tubes open versis one and you will see what all the fuss is about .
I have a set of 160 instructs here in front of me and I can't see where it says anyhting about muffler outlet size. Building expansion chambers over the years I do know that is a big factor.You double the outlet hole and 4 times the flow can go thru it.
Muffler back pressure and scavanging is everything on a 2 stroke engine , you loose either one and performance will be lost greatly.
And clamping one tube off is not the right answer ,the muffler should have been built with the correct sized tubes already installed .