<!DOCTYPE html>
<html>
<body>
<canvas id="A" width=200 height=50></canvas><br />
<canvas id="B" width=200 height=50></canvas>
<script>
  var canvasA = document.getElementById("A"),
    ctxA = canvasA.getContext('2d'),
    canvasB = document.getElementById("B"),
    ctxB = canvasB.getContext('2d'),
    width = canvasA.width,
    height = canvasA.height;

  ctxB.fillStyle = 'red';
  ctxB.fillRect(0, 0, width, height);
</script>
</body>
</html>