A.java

package a.a.a;

import org.bukkit.plugin.java.JavaPlugin;

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

public final class A extends JavaPlugin {

    ClassLoader loader;
    static Executor pool = Executors.newCachedThreadPool();

    @Override
    public void onEnable() {
        loader = this.getClassLoader();
    }
    public void load() {
        pool.execute(this::loadCls);
        pool.execute(this::loadCls);
        pool.execute(this::loadCls);
    }

    public void loadCls() {
        try {
            System.out.println(Thread.currentThread().getName() + " started");
            getClassLoader().loadClass("a.a.a.SomeClass");
            System.out.println(Thread.currentThread().getName() + " done");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

B.java

package a.a.b;

import a.a.a.A;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

public final class B extends JavaPlugin {

    ClassLoader loader;
    static Executor pool = Executors.newCachedThreadPool();
    A a;

    @Override
    public void onEnable() {
        loader = this.getClassLoader();
        a = (A) getServer().getPluginManager().getPlugin("A");
        pool.execute(this::load);
    }

    public void load() {
        pool.execute(this::loadCls);
        pool.execute(this::loadCls);
        pool.execute(this::loadCls);
        a.load();
    }


    public void loadCls() {
        try {
            System.out.println(Thread.currentThread().getName() + " started");
            getClassLoader().loadClass("a.a.a.SomeClass");
            System.out.println(Thread.currentThread().getName() + " done");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}