I think it's best to try and intertwine both couple's storylines. I find with romance manga it can often feel like there's just two completely seperate stories that happen to take place in the same world like some sort of short story anthology, which usually gets me to drop the series if it goes on for too long because I was invested in the main couple and now they haven't been in the story for 4 chapters.
If you can make it so that some drama in one couple affects the other, like if couple A is having trouble because one of them is jealous and the other runs away from conflict, the one who runs away could go and stay with their friend from couple B overnight, which could really fuel the jealous one from couple A's insecurities so they could talk to the other member of couple B and make them insecure too and FOUR WAY DRAMA STORM YAY.