"For the determination of the vortex strength (circulation) of the An-2 biplane it is assumed that the wing tip vortices of the upper and lower wing are merging relatively fast since the distance between the wings is not large. Hence circulation is determined in the same way as for a monoplane.

...

It has to be noted that the airspeed of the An-2 for this flight phase is considered to be relatively small for an aircraft of that size, which can be attributed to the configuration with two wings and accordingly a comparatively large wing area. The relatively small airspeed is contributing to a comparatively large circulation for an aircraft of that size."

http://www.dglr.de/publikationen/2014/340177.pdf