How to Instantiate a Combat Manager to handle attack order?

So In this game, when 2 enemy units collide, they enter melee combat. The game is RTSish, so any number of melee combats could be occurring simultaneously. When in combat, I want to ensure that units do not just hack away at each other, but rather that when one is attacking, the other “receives” the attack (in order to run a blocking animation etc.)

My idea here, is to have a “combat manager” that essentially contains a list of attacks. The manager would build this list based on unit attack speed, so that when a unit is ready to attack, it adds an action to the combat manager. Then the combat manager, pops the top action off the list, listens for it to resolve, then pops the next, and so on.

My question is, when two units run into each other, how do I handle instantiating an instance of this combat manager for that specific melee? I had though to call it on the units “EnterMelee” method, but how do I ensure that both units don’t call a combat manager, and that they instead share one?

I’m writing this in c#, but i’m looking for more conceptual advice than straight code.

I like high-level managerial classes, so I’d make mine a singleton, and let it resolve Conflicts. Each agent entering conflict would signal the manager. The method arguments would include a reference to the agent and its combatant. If there’s no conflict in the list between those two agents, create a new Conflict object.

A calls with (A,B);
Manager creates Conflict;
B calls with (B,A);
Conflict already exists so do nothing;

A loop on the manager runs to resolve active conflicts. The order-of-attack and other mechanics are thus separated from the lets-do-combat logic. The Conflict object could examine whatever logic necessary, have its own timers, whatever’s needed.