how to use 2 canvases

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HTML5

Canvas

I want to draw analog clock. So the main canvas will be dial and, hours and minutes arrows, and the second will be seconds arrow (for update every 1 second).

HTML

<canvas id="clock"></canvas>
<canvas id="second"></canvas>

JScript

canvas = document.getElementById('clock');
canvas.width = document.documentElement.clientWidth;
canvas.height = document.documentElement.clientHeight-6;
canvas = document.getElementById('second');
canvas.width = document.documentElement.clientWidth;
canvas.height = document.documentElement.clientHeight-6;

But canvases are not transparent and I can not to draw. Any ideas?

Thank you for attention.