This is probably your best bet.

As you noticed, having multiple canvases does not impact memory usage that much since much of the memory is shared between multiple instantiations of the same widget.

The only other optimization I can think of is that you don't need to invoke the third party code twice. What you can probably do is invoke it once to get the object drawn on one canvas, then grab the coords of the new object (using the coords() method), and draw it on the second canvas. Probably a bit faster.

HTH.