It is not literally possible to read more than one of the multiplexer’s pins at once, but the loop happens so fast that you can turn the two (or more) pots at once and it will read them in turn but fast enough to act like they are all being read at once. (if it is still not quite fast enough look at removing or shortening the delay(10) in each of the ‘read’ code blocks in the sketch.)
hope this helps, a