tournament_contest_versus_pair_effect

tournament_contest_versus_pair_effect = {
 	# Remove any event score gained in last round
	every_guest_subset = {
		name = $STAGE$
		phase = tournament_phase_$CONTEST$
		add_to_list = valid_contestants
		set_variable = {
			name = progress_to_victory
			value = progress_to_victory_active_contestant_value
		}
	}
	if = {
		limit = {
			any_guest_subset = {
				name = $STAGE$ # e.g. quarter
				phase = tournament_phase_$CONTEST$
				count < contest_versus_$STAGE$_count
			}
		}
		debug_log = "Not enough versus contestants"
		while = {
			limit = {
				any_guest_subset = {
					name = $STAGE$ # e.g. quarter
					phase = tournament_phase_$CONTEST$
					count < contest_versus_$STAGE$_count
				}
			}
			if = {
				limit = {
					any_guest_subset = {
						name = contestant
						phase = tournament_phase_$CONTEST$
						save_temporary_scope_as = new_list_member
						NOT = {
							scope:activity = {
								any_guest_subset_current_phase = {
									name = $STAGE$
									this = scope:new_list_member
								}
							}
						}
						activity_is_valid_tournament_contestant = yes
					}
				}
				random_guest_subset = {
					name = contestant
					phase = tournament_phase_$CONTEST$
					limit = {
						save_temporary_scope_as = new_list_member
						NOT = {
							scope:activity = {
								any_guest_subset_current_phase = {
									name = $STAGE$
									this = scope:new_list_member
								}
							}
						}
						activity_is_valid_tournament_contestant = yes
					}
					save_scope_as = backup_contestant
				}
			}
			else = { tournament_versus_create_contestant_fallback_effect = yes }
			add_to_guest_subset = {
				name = $STAGE$
				target = scope:backup_contestant
				phase = tournament_phase_$CONTEST$
			}
		}
	}
	# Pair remaining contestants
	while = {
		limit = {
			any_guest_subset = {
				name = $STAGE$
				phase = tournament_phase_$CONTEST$
				NOT = { has_variable = contest_$STAGE$_match_$CONTEST$ }
			}
		}		
		ordered_guest_subset = {
			name = $STAGE$
			phase = tournament_phase_$CONTEST$
			limit = {
				NOT = { has_variable = contest_$STAGE$_match_$CONTEST$ }
			}
			order_by = {
				value = 50
				if = {
					limit = { scope:activity = { has_current_phase = tournament_phase_joust } }
					add = contest_horse_aptitude_value
				}
				else_if = {
					limit = { scope:activity = { has_current_phase = tournament_phase_duel } }
					add = contest_foot_aptitude_value
				}
				else_if = {
					limit = { scope:activity = { has_current_phase = tournament_phase_board_game } }
					add = contest_board_game_aptitude_value
				}
				else = { add = contest_pugilism_aptitude_value }
			}
			# Save pair of contestants
			save_scope_as = versus_matchee
			scope:activity = {
				random_guest_subset = {
					name = $STAGE$
					phase = tournament_phase_$CONTEST$
					limit = {
						NOR = {
							this = scope:versus_matchee
							has_variable = contest_$STAGE$_match_$CONTEST$
						}
					}
					weight = {
						base = 1
						# Prefer to leave rivalry matches later
						modifier = {
							factor = 0.5
							has_any_bad_relationship_with_character_trigger = { CHARACTER = scope:versus_matchee }

As written in game/common/scripted_effects/04_dlc_ep2_tournament_effects.txt.