A rotary cutting deck for a mower has dual, side-by-side cutting chambers that discharge their grass clippings through a common, rearwardly extending exit tunnel. A pair of pivotal flow control baffles are positioned ahead of the mouth of the exit tunnel. The baffles have an open position in which the baffles clear the mouth of the exit tunnel and form guide channels in concert with some of the walls of the cutting chambers which help direct the grass clippings from each cutting chamber towards the mouth of the exit tunnel. The baffles can be pivoted to a closed position in which the baffles block but do not completely close the mouth of the exit tunnel to provide the operator with some way of controlling how long it takes to fill a grass collector attached to the exit tunnel. A mulching plug with a forwardly extending separator vane can be inserted into the mouth of the exit tunnel to convert the deck from a discharge/collection mode of operation to a mulching mode of operation.