Sounds like you need 2 variables: a counter, and the last button pressed.  I'll assume this is a WinForms application, since you did not specify at the time I am writing this.
class MyForm : Form
{
    // From the designer code:
    Button btnX;
    Button btnY;
    void InitializeComponent()
    {
        ...
        btnX.Clicked += btnX_Clicked;
        btnY.Clicked += btnY_Clicked;
        ...
    }
    Button btnLastPressed = null;
    int counter = 0;
    void btnX_Clicked(object source, EventArgs e)
    {
        if (btnLastPressed == btnY)
        {
            // button Y was pressed first, so decrement the counter
            --counter;
            // reset the state for the next button press
            btnLastPressed = null;
        }
        else
        {
            btnLastPressed = btnX;
        }
    }
    void btnY_Clicked(object source, EventArgs e)
    {
        if (btnLastPressed == btnX)
        {
            // button X was pressed first, so increment the counter
            ++counter;
            // reset the state for the next button press
            btnLastPressed = null;
        }
        else
        {
            btnLastPressed = btnY;
        }
    }
}