I just build a canister muffler for a 72cc engine.
I used 2-5/8 id tubes and the engine was choking out.
I installed the third pipe and now it runs pretty good.

2 hole muffler: http://www.rcuvideos.com/item/BMV2KV388GZSF0NZ

3 hole muffler: http://www.rcuvideos.com/item/GYZZ4BDXYD2G4VVP

If you only use 2 exhaust tubes they will have to be bigger than 5/8

Any way that is what I found.

By the way I just finnished a 61cc Echo Twin conversion
It does use 5/8 tubes and it does run good.
http://www.rcuvideos.com/item/8PXSLQ2FFLSVFK62