Yup. It's possible to have two people play on one mac. I prefer using just one copy of the Ho!, but you can do it with two. You *MUST* make certain that the "automatically end turn for unconnected players" is OFF when you setup the game.
Create a new game of my choice with "automatically end turn for unconnected players" off.
Then I enter the game. I usually play as "Casper" and without a password.
Then after that, immediately CLOSE the game.
Go to OPEN. It will ask you for your name & password. Here, add the new person. In my test cases, I'm "Ghost" without a password (I trust myself).
Then repeat as neccessary to add however many players you want (max 20 with computer opponents).
Then click "Everybody's ready"
It should now be the first game turn for the last player added.
Play the turn.
When done, go to the FILE menu and select "END TURN and SWITCH PLAYERS"
Have the next person play their turn.
repeat until you've kicked butt.
When running multiple copies of Ho! on one mac, you just need to wait at the "I'm playing alone" or "Everybody's Ready" dialog for all the players to join. You still need to create the galaxy with "Automatically end turn for disconnected players" OFF. That option is only useful for network games in case someone gets disconnected from the network so everybody else can still play.