class Cls_1 : public QThread
{
    Q_OBJECT

public:
    Cls_1();
    virtual ~Cls_1(){};

    void Method_1(const double &param);

private:
    void run();
};

Cls_1()::Cls_1()
{

}

void Cls_1::run()
{
    ...

    exec();
}

void Cls_1::Method_1(const double &param)
{
    ;
}

class Cls_2 : public QObject
{

 Q_OBJECT

 public:
    Cls_2();
    virtual ~Cls_2(){};

    void Method_1(const double &param);

 signals:
    void SignalMethod_1(const double &param);
}

Cls_2()::Cls_2()
{

}

void Cls_2::Method_1(const double &param)
{
    emit SignalMethod_1(param);
}

Cls_1 *obj_1 = new Cls_1();
obj_1->start();

_sleep(10000);

Cls_2 *obj_2 = new Cls_2();

QObject::connect(obj_2, &Cls_2::SignalMethod_1, obj_1, &Cls_1::Method_1, Qt::ConnectionType::DirectConnection);

obj_2->Method_1(1);