Guys, I see that this promt is kinda hard to find, so let's make it easier. Main couple may be interpreted as "MC+MC" or them individually before/after getting together
Let's take first interpretation. That means they shouldn't be involved in same LT together, so if one of them is involved in LT individually, it's okay
Example: there is second leads couple, where girl is in love with male lead. Female lead doesn't like male lead at that point and doesn't care about his relationship. So, main couple isn't involved in LT