saddlebag windowboxes

In my transient nyc life I'm always looking for simple DIY home items that don't need any kind of hardware/installation. I was thinking about a windowbox setup where two identical windowboxes are joined together by a series of strong pieces of wire, each a foot long or so (modify wire length according to window/sill depth), so one box hangs outside the window as usual and the other box hangs inside the window. In the outside box you grow plants that can tolerate outdoor conditions, and in the inside box you can grow plants that are more delicate (can't stand up to your wind/rain conditions), don't need as much sun, etc. The set of wires is cheap, flexible so the window can close, and strong so the boxes stay hooked together even when they're heavy with water/rain.