First you need to get a copy of the terminal SETI client.
Now make a folder in you home directory called for example "SETI", then in that make 2 further folders, one called "CPU1" and another called "CPU2".
Now copy the "setiathome" file into "CPU1" and make a duplicate in "CPU2".
So now you should have a folder in your home directory called "SETI", in that should be 2 folders. one called "CPU1" and "CPU2", each with a copy of the "setiathome" file inside.
Now start off a terminal session and change directory into the "CPU1" folder:
Example:
cd seti/cpu1
then type:
./setiathome -verbose
Now open another terminal session and do the same again but instead of changing into the "CPU1" directory, change into "CPU2".
And that should be it, your mac will be working over time working on all those units!