What counts as the wingspan of a tandem-wing aircraft?
Because wetted aspect ratio $b^2/{S_w}$ merely improves on wing aspect ratio as an estimate of efficiency, for $b$ just choose whichever wingspan is largest.  That's good enough to compare one tandem planform to another.
You could get fancy and use a weighted average of the wings' spans, weighted by how much lift each one produces, with those in turn weighted by time spent in climb, cruise, and descent... but that seems against the spirit of a quick estimate provided by two easily measured numbers.