You can try filling the whole canvas first, then use composite mode to knock out the shapes.
Example:
ctx.fillStyle = "rgba(0, 0, 0, 0.5)";
ctx.fillRect(canvas.width, 0, 0 - canvas.width, canvas.height);
// next shape will punch hole in the draw rectangle above
ctx.globalCompositeOperation = 'destination-out';
ctx.fillStyle = "rgba(0, 0, 0, 1)"; // make sure alpha is 100%
ctx.beginPath();
ctx.moveTo(300, 60);
ctx.arc(300, 60, 50, 0, 2 * Math.PI);
ctx.moveTo(500, 160);
ctx.arc(500, 160, 50, 0, 2 * Math.PI);
drawEllipse(ctx, 103, 23, 100, 30)
drawEllipse(ctx, 503, 23, 100, 30)
ctx.fill();
ctx.globalCompositeOperation = 'source-over'; // reset comp. mode
Alpha (not the color) will determine how much visible the hole will be.
Hope this helps.