How to detect two key inputs at the same time?

You need to add an eventhandler-method to the KeyDown-event of the control(s) for which you want to recognize this. You can do this either through the Designer via Properties > Events or manually. Example here for a TextBox-Control:

// if you want to add it "manually" then place this
// line in the constructor of the form:
SomeTextBox.KeyDown += SomeTextBox_KeyDown;

private void SomeTextBox_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Z && e.Modifiers == Keys.Control)
    {
        // do something here
    }
}

If you want to process this event for more than one control, you can add the same eventhandler-method to the events of many controls:

SomeTextBox.KeyDown += SomeControl_KeyDown;
SomeOtherTextBox.KeyDown += SomeControl_KeyDown;
SomeOtherControl.KeyDown += SomeControl_KeyDown;

In that case, if you want to know from which control the event originated, you have to look at the argument sender which will be a reference to that control.

If you want to receive that event also for the Form itself you have to set its property KeyPreview to true.