Well, the biggest challenge that you’re going to face with that is developing enough tension between the two characters to make the reader worry/pick sides/care. The entire reason behind why fights between two characters are imbalanced is that it’s an easy way to generate tension, even if it’s just one character thinking that they are less capable than their opponent.
Between two characters who know that they are on equal footing in terms of skill (as opposed to the author knowing that they are on equal footing, but they don’t) is that they may each prepare for the fight differently. If the characters both just say “well, we’re on equal footing so I’ve got a 50/50 chance” and then do nothing about it, well, that’s sort of meh, isn’t it? As opposed to characters who say “well, we’re on equal footing, my shot at victory is 50/50, let’s see what we can do to improve those odds” and strategize, even though they might not have to. These characters become active and are active participants in the approaching battle.
Here are some easy techniques you can do:
Instead of focusing on how equal the two characters are, focus on their combat strengths and weaknesses instead and how they match up against each other. This will give the reader the impression that while Protagonist X is a really solid fighter, Antagonist B can still get him/her/it through either a character flaw or a stylistic one.
If the two characters use different styles for combat, focus on how those styles match up against one another. Protagonist X is really good at Muay Thai, and can definitely win if they can keep their opponent at range and stay on their feet, but could be screwed if Antagonist B, who is great at Sambo, takes him/her/it to the ground.
By playing up their strengths and weaknesses, you build natural tension by providing a real, solid chance for failure. The chance for failure is important, because it’s part of how we get the reader invested in the story, so that they’ll keep reading to see how it all turns out (or if your protagonist is annoying, keep reading on the hope that there will be some vindication via pummeling).
Always note the character flaws. For example: going into an even match, a self-destructive character’s greatest enemy is themselves.
Keep track of the stakes. With the potential for failure comes the potential cost of failure, high risk is high reward. Keep the audience informed on what the consequences are or what the character believes the consequences will be if they lose. Also, know what the cost will be for winning. We always give something up for victory, know what that will be. Even the friendliest, evenly matched bout can be nerve-wracking if there’s something else at play underneath the surface. Whether it’s something as small as relating to the character’s self-esteem, or something as big as the worry that the compulsive gambler’s compulsive gambling habit will leave the party stranded on a desert island without transportation, the stakes are what make the game. Remember, there’s always something to lose, even for characters who believe that they have nothing else of value.
The fight should have meaning. Every sequence, event, character, and written line in a story must earn it’s place there. So, you have to make it count, have it show something about your characters or be a stepping stone to something greater. The fight needs to fit within the story’s themes along with the events leading up to it and the actions the characters take afterwards. So, sit down and figure out why the fight needs to be there. What is going on. What it will mean to the characters and their development. Focus on how it will change not only them, but the secondary characters who surround them. How do those characters react to the combatants actions? Do some approve? Are some horrified? How do they feel? How does it affect them?
Tension is key to making any fight work, the part where it leads somewhere is how you keep the reader engaged. Focus on what the combatants are good at, what they’re bad at, and what they’re doing to make up the difference. Active characters are important.
Try these ideas out, hopefully they will help.