When one person’s Mercury aspects another person’s Mars, stimulating conversations and challenges to each other’s ideas are common.
The hard aspects (square, opposition, and quincunx) can cause conflict in the partnership. The Mars person might interrupt or aggressively debate with the Mercury person. The two of you may challenge each other’s ideas, which can leave either one of the people feeling attacked. The Mars person may try to force his opinion on the Mercury person, while the Mercury person may be overly critical or nit-picky over the Mars person’s ideas. It is difficult for the two of you to communicate in a calm and rational way, which can make living together very difficult.
The harmonious aspects (sextile or trine) are far easier to handle. In this case, the Mars person energizes the Mercury person’s mind. Conversations are lively, and there is a healthy amount of intellectual competition between the two of you. You work well together, and enjoy tackling intellectual games with one another. The two of you are fond of debating with each other.
The conjunction can go either way. If the conjunction receives flowing aspects from either person’s planets, the aspect will manifest as lively, stimulating conversations with a healthy amount of competition. If the conjunction receives hard aspects from other planets, petty arguments and intellectual battles may plague the relationship.
My ex-boyfriend and I had a Mercury-Mars conjunction in Aquarius. As such, we would often engage in debates, and they tended to get quite heated from time to time! It never got out of hand, though; the conjunction received only trines and sextiles from our other planets. As the Mars person, I stimulated his mental energies; I showed him different ways of looking at things (Aquarius), which opened his mind. There was quite a bit of competition between the two of us when it came to debating, but I never ended up hating him for it. We were also very competitive when it came to playing games or sports.