The first swarm you saw was the prime swarm with the old queen.  The other was
probably a "cast" with a virgin.  Sometimes if the colony is strong enough and
the bees have the genetic predisposition several casts will emerge before that
colony's swarming season is over.
Don't be too worried about swarms.  They are the fun part of beekeeping and to
be looked forward to.  The easiest way of swarm avoidance is to use one of the
many ways of making an artificial swarm.  Do it for them before they do it for
themselves.
Chris Slade