Both Sun are in the same sign, unless one is at the late degrees of a sign and the other is at the early degrees of the following sign.
If the Suns are in the same sign, they have the same energy, essence. There is common ground, many traits in common. This can help in understanding each other which certainly helps any relationship. There’s the adage “Familiarity breeds contempt”. If you are similar to another person, you could say that you are familiar with the ways of the other person. Hence, being with someone of the sign sign can sometimes lead to conflict. That is why this is a hit or miss aspect to me, especially when it comes to marriage or long-term romantic relationships.
I suppose the potential for marriage depends on other interaspects. I suppose if I have to prioritize, I would place more weight on interaspects between the Sun, Moon and Ascendent as they are the astrological holy trinity after all. But in reality, you would look at all tight interaspects as they have a bearing on the relationship.