Run two instances of prime95. Set the affinity for one to "cpu 0" and the other to "cpu 1". This can be done in the settings within the program. To run the second instance go to a command prompt and switch to the prime95 directory. Use the -a command line switch to run it.
example:
prime95 -a2
or you can just start both instances from a command line.
prime95 -a1
prime95 -a2