I wonder if this would be a clever way of hinting to the reader that a new relationship is growing, while also showing the love the main character still has for their first relationship, without making the new 'ship feel like it's a bad copy of the first. Instead, like a love that works out this time.
To paraphrase, how will this particular scene 'read'? And how to make it feel important, like the lead into a subplot that builds this relationship?
First, for the sake of a coherent answer I am assigning your characters arbitrary pronouns:
- the MC is 'he'
- the 2 loves are 'she'
–– but my answer does not change at all if the pronouns are re-arranged.
Signaling a Love Rival
Without reading your scene, I feel that you'd be signaling a comparison to the old relationship, which might feel like a promise to the reader this old love will somehow return and become a rival to the new love. Especially if the first love actually is perfect.
At the very least, you are signaling that 'he' is not over the first love since the new experience is going to be directly compared. A ghost can be a more powerful love rival because that person doesn't update or contradict the memories.
So unless he's ready to dump his ideals on the spot, the 'beat for beat' comparison does not work out in the new girl's favor, especially to hear him tell it. She might be perfectly fine for someone else, but really no one can compare to his ideal.
The first romance was naive
The rules of storybook love (if there is such a thing) imply there is only one 'true' love. In my opinion, I think you might want to hint at flaws in the innocent love (beyond the arranged marriage).
Something about this walk triggers this specific memory, however the reader can see there is something false with his naive ideal. Not 'duplicitous' but just naive. He's held on to the general memory of 'her' as enchanting and wonderful, but upon examining this specific memory now, it was not quite as ideal as he remembered –– perhaps it was actually one-sided, which the reader can recognize would not have been good for the MC, or he is a lower caste, or her behavior had an ulterior motive which a naive boy would not recognize, but an adult with more experience would see through. She doesn't need to be a villain, but maybe a bit selfish, or intentionally helpless, or condescending....
He doesn't need to fully process this memory at this time, it's enough that the reader sees it was not quite as perfect as he wants to believe.
There is subtext that she had an arranged marriage to someone else, ergo her guardian did not consider the MC an ideal match. If she was aware of the arranged marriage she may be leading him on, or fooling herself. There may already be a reason this relationship was never going to happen, so she does not need to take the blame for bad character. Both of them imagining it could be any other way is 'naive', and the fact that she went through with the marriage proves it.
If she returns later in the story, foreshadow how she will behave as an adult. If the is manipulative, she will manipulate. If she feels the duty to sacrifice personal happiness for her family, she will still have this same moral compass, however badly she has been treated by it. If she truly loves him, she may also carry a naive ideal about him into adulthood.
The 'true' romance is a challenge
The new partner is not a romantic prospect yet. Again the reader should glimpse something the MC is not ready to acknowledge. He does not think of 'her' in that way at all.
Remember: a romantic subplot is still a subplot. It must have a conflict with antagonisms and setbacks (hopefully entwined with the main plot), and the protagonist needs to earn the win only after correcting his own mistakes. Without these conflict beats the romance is not really a subplot, it's a hookup.
She may be the complete opposite to his naive ideal: bossy, un-feminine, un-interested in upholding courtly ideals. She may be lower caste, a warrior with a lothario-reputation. She talks back, she's not serious. In fact, she's often annoying.... You get the idea, it doesn't need to go that far. Maybe he never noticed her at all.
The trick for you as author is to use subtext and their dialog to show us she is getting under his skin. She is challenging his naive preconceptions, by being true to her character and not trying to be someone's ideal. Maybe she is just competent and an equal, and that doesn't add up to love for him. Maybe he is the lothario drowning his broken heart in easy sex, and she challenges him by setting up boundaries.
I'm not trying to write your story, but there should be internal conflict preventing him switching romantic focus. She can't be a stand-in for the 1st, and she can't be a rebound. If it's too easy, the reader will doubt it's the true love.
Whatever their dynamic, it is on display in this scene. If they are heated, they will be arguing. If they are like-minded, they are answering each other's sentences. You are foreshadowing their relationship by giving them an unusually candid exchange from the start.
They might be taken off-guard by their instant chemistry, causing them to drop some formalities. Do give readers a taste of them at their best, but they sabotage it. They have no relationship history at this point, so they can have a lot of misconceptions about each other. It's normal to pull back after getting a bit too close.
Since he can't switch romances too quickly, it might be ok to make it clear she is interested and pursuing him. But that could change if he admits she doesn't compare to his ideal (or he explains his ideal for too long). There are a hundred ways he could sabotage this first date, but it should be an aspect of this hang-up for the one that got away.
In the comparison between the 1st and 2nd love, #1 remains the protagonist's choice but #2 needs to be the reader's choice. 'She' will need to stand her ground and be more interesting than his ideal. Readers should not be satisfied with how this 1st date ended. They should feel it was a missed opportunity, and that the characters egos got in the way of a good thing.
Build the subplot
Whatever the details, you're creating an inciting incident for this relationship, a couple of antagonisms (mostly their ego and pride), and a conflict that the first meeting went badly.
You now have the space to build their relationship from scratch, while they already have a too intimate closeness that throws sparks. It could be completely lop-sided, unacknowledged, or obvious to everyone –– but it isn't easy-peasy instant relationship.
By the time they are finally ready to commit and be happy together, that's a good time for Miss Perfect to return needing help, and throw a monkey wrench into the hard-earned relationship.