In Astrology, Mercury represents our communication style, how we process information, and our logical mind.
When your Mercury is conjunct your partner’s Mercury, your thinking processes are very similar. You are likely to hold similar opinions and viewpoints. Your communication is excellent, and misunderstandings are unlikely. You are always on the same page, and quickly understand where the other person is coming from. On the downside, this aspect can indicate heated debates. Because your partner’s Mercury matches yours, you may feel you have found your intellectual match.
When your Mercury trines or sextiles your partner’s Mercury, your thinking processes are different, but complementary. You love talking to one another, and hold similar opinions and views on the same subject. Again, misunderstandings are unlikely.
Mercury square Mercury is a very difficult aspect to deal with. This indicates a difference in opinions or perspectives. Debate and mental sparring are likely, but you may frustrate each other when you voice your opinion. You are likely to think the other person is just wrong! This aspect can make it so the two of you feel you do not really get along.
I had this aspect in one of my previous relationships. My Mercury is in Capricorn, and his Mercury is in Aries. He would get annoyed because he felt I was too deliberate with my speech, while I found him too impulsive. I did not enjoy the fact that he tended to speak before he thought, and he disliked that I always thought before I spoke! It got to the point where I could not even stand speaking to him, so we broke up.
The opposition between Mercuries in Synastry is less difficult than the square. In this case, you and your partner have opposing, but complementary, views and opinions. Your partner forces you to see the opposite viewpoint on a matter, which can be mentally refreshing for the both of you. On the downside, this aspect can indicate misunderstandings and disagreements, which can lead to frustration. You may feel you don’t agree with each other on anything, and have no common ground.