Beastmaster - Add 2nd companion
 

How do I add a second companion for a player's beastmaster dedication?

On the Ranger tab, Click on "Add more Animal Companions, Eidolons, or Familiars"

And since you used the word "Dedication", if this is a PF2 question posted in the PF1 space, add the "Additional Companion" feat - it's in the 4th level feats.

Also, on the play tab, there's a "Minions" section in the left-hand column - there, you can enable/disable which companions are active at this time.