I like to use 2 speakers with my MacPro. I am able to get one speaker to work. I use a dj software program to play music. I am not a dj - play music in my home for guests/self. I use a powered speaker Gemini, RCA cables - I love it altho the speakers have some feedback. I like to purchase another speaker, but before doing so, I like to know whats the best way to hook up the second speaker. I see djs with MacPro using an establishment's speakers - like to know if I took my Mac to a location how would I hook up the Mac to their speakers?
Whenever I come across someone using an apparatus I think I may want to learn, I make it a point to simply walk up to the person and ask them HOW did they manage to do this, that and the other thing, you would be surprised how friendly people are when asked a question about their hardware setups. Not to mention saving some cash in the long run buying exactly what you need to get the task done, rather than spending on hardware you THINK you may need.
What type of speaker are you using? You should be able to connect it all very easily through the headphone jack.