Cross-talk of condition_variables signal and how to avoid it

Question

I'm using the mutex and condition_variable pair to implement mult-threaded processing. I have read examples and solid explanations like this and that. However, I do not understand why separate variables trigger each other. For example,

mutex alert0, alert1;
condition_variable var0, var1;
void toy0() {
    std::unique_lock<std::mutex> lock(alert0);
    var0.wait(lock, [=] { return true; });
    cout << "Toy0 triggered" << endl;
}
void toy1() {
    std::unique_lock<std::mutex> lock(alert1);
    var1.wait(lock, [=] { return true; });
    cout << "Toy1 triggered" << endl;
}
void main(){
    std::thread t0 = std::thread([=] {
        toy0();
        });
    std::thread t1 = std::thread([=] {
        toy1();
        });

    {
        std::unique_lock<std::mutex> lock(alert0);
        var0.notify_all();
    }
    t0.join();
    t1.join();
    return;
}

yields

Toy0 triggered
Toy1 triggered

If this is the intended outcome, how may I have cross-talk-free signals in different parts of the program so each wait() can be triggered by a specific condition_variable's notify_all(), not others'?