PANEL* pan_one =
{
  ...
}

PANEL* pan_two =
{
  ...
}

...

if( ( pan_one.pos_x + pan_one.size_x >= pan_two.pos_x ) || ( pan_one.pos_x <= pan_two.pos_x + pan_two.size_x ) ) // overlapping in x dimension
  if( ( pan_one.pos_y + pan_one.size_y >= pan_two.pos_y ) || ( pan_one.pos_y <= pan_two.pos_y + pan_two.size_y ) ) // overlapping in y dimension
  {
    // now we know both panels are overlapping in both the x and y dimension!
    // now destroy panels, etc.
  }