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I assume it's something like setting the order in which heroes appear from the very beginning of the game. If a hero of a particular class isn't available in the tavern, the next hero in the order is taken.

What does "not suitable class" mean? At the beginning of the week, a free hero of the corresponding race is placed in the left slot. In the right slot, and in the further queue of both slots, are free heroes of any class.
It is implied that the game can have mirror pairs of any heroes at the same time, including for the AI. That is, it is necessary to increase the number of heroes by two times, or to remove half of the heroes from the game. In the case of removing heroes, there may simply not be enough of them.

Essentially, increasing the number of heroes by two seems to be the main technical problem.

Added 7 minutes later

The hero pool will be duplicated - there will be two of each.

A hero can only appear in the tavern queue if both mirror players have it at the same time, unless it's a split. That is, if one of the players has it in the game, it cannot appear.