document.getElementById("demo").onmouseover = function() {mouseOver1()};
document.getElementById("dd").onmouseover = function() {mouseOver2()};
document.getElementById("demo").onmouseout = function() {mouseOut1()};
document.getElementById("dd").onmouseout = function() {mouseOut2()};

function mouseOver1() {
  document.getElementById("demo").style.background = "blue";
  document.getElementById("dem").style.visibility= "visible";
}

function mouseOut1() {
  document.getElementById("demo").style.background = "black";
  document.getElementById("dem").style.visibility= "hidden";
}

function mouseOver2() {
  document.getElementById("dd").style.background= "green";
  document.getElementById("dem").style.visibility= "visible";
}

function mouseOut2() {
  document.getElementById("dd").style.background = "black";
  document.getElementById("dem").style.visibility= "hidden";
}