Why not create 2 broadcasts. One for each group!
That is possible but just trying save some key strokes if we could, Only sending one message and updating the one message. If that can not be done then yes we would have to send two broadcast.
Alternatively, you can review Account based assignment, where you can send a broadcast to users of multiple Accounts.
And of course, you can use the "Actions->Clone" feature to auto populate most of the fields - certainly not the best of the options but will save you some keystrokes