The Mercury person can help the 12th House person gain insight into the mechanisms and patterns of their subconscious mind. You can stimulate each others imagination. There can be a mutual interest in art, music, psychology and mysticism. The 12th House person can bring out the intuitive insights of the Mercury person. There could be difficulties in communication, deception,confusion if there are other bad aspects towards mercury. On the worst side this could be manifested as escapist tendencies which includes morose and neurotic fantasies.