I have a whole theory about this, maybe it's too long and you don't care, then you can just skip this part hehe.
Theory
////Most romance watchers are women, hence, the Female Lead is actually the Main Character, the one we empathize with, and the Male Lead is the Prize/Quest/damsel-in-distress. Basically, in most cases, the female lead will have the most screen time, we follow her from when she wakes up to when she goes to sleep, whereas the male lead is just there to basically be pretty and fall in love with her after she's conquered any obstacles the writer chose to throw her way. Which is why we often have Second Lead's syndrome. The SML basically being the alternative choice, the fork in the road. Having Second Male lead syndrome is akin to turning your head in the street when the babe passes you with your actual honey boo at your arm.
This explains why there are not so many 2FL's syndrome. The 2FL is usually at worst the antagonist/villain, at best she's the sidekick/helper. Even when the FL is a jerk, rarely do we deeply care about the ML (AKA the prize) being happy, we only care about whether the protagonist wins her quest. It takes a very talented writer or a really bad one, to give one 2FL's syndrome, in my opinion. /////
So here a few recommendations.
The first that comes to mind is To Be With You . This cdrama is frankly a hidden gem, that has reconciled me with chinese romance dramas (particularly those on the longer side), it's so good. There are two potential FLs in this drama, and one of them starts off real bad before turning good, so good. Honestly, there was a point where I thought, if the main leads don't get togeter, this 2FL should have the guy LOL.
First Love: Hatsukoi she was such a good person, especially to him.