How to properly use QGridLayout to addWidgets next to each other in Qt5

I have a small problem trying to put two layouts next to each other.

The actual result I have so far is the followig:

But it should look like this one below:

What am I missing?

Below the small snippet of code I am using for this:

MyViz::MyViz(QWidget *parent) : QWidget(parent)
{

//  Qt Components set up......

QGridLayout* control_layout = new QGridLayout();
control_layout->addWidget(thickness_label, 0, 0);
control_layout->addWidget(thickness_slider, 0, 1);
control_layout->addWidget(cell_size_label, 1, 0);
control_layout->addWidget(cell_size_slider, 1, 1);

control_layout->addWidget(thickness_label_second_renderer, 0, 2);
control_layout->addWidget(thickness_slider_second_renderer, 0, 3);
control_layout->addWidget(cell_size_second_renderer_label, 1, 2);
control_layout->addWidget(cell_size_second_renderer_slider, 1, 3);


// construct lay-out and render panels next to each other
render_panel = new rviz::RenderPanel();
render_panel2 = new rviz::RenderPanel();

QVBoxLayout* main_layout = new QVBoxLayout;
main_layout->addLayout(control_layout);
main_layout->addWidget(render_panel);
main_layout->addWidget(render_panel2);

// set the top-leve layout for this MyViz widget
setLayout(main_layout);

// other operation....

}