2 layers of plastic film window insulation?
There are, as always, tradeoffs. First, 1 inch is more than is generally considered ideal - 1/2" or so is preferred as it's less prone to internal convection currents. Yes, plastic window film kits are often installed with considerably larger spacing, but they are also commonly installed on less than ideal windows where they stop actual drafts...

Any multi-pane system comes with a very basic tradeoff - more panes, less light. Neither glass nor plastic lets all light through, and multiple panes/layers have a multiplicative impact on the light transmission - if a single pane lets through 90 % of the light, two let through only 81% and 3 let through only 73%...

The additional layers do provide improved insulation value over a single pane, though even a triple-pane system is still a poorly insulated area compared to the rest of the wall (if the rest of the wall is properly insulated, anyway.)

I suspect hard data on the effectiveness of plastic film layers would be difficult to source (though I could be wrong, I don't feel like looking for it right now based on that suspicion) but you'd probably see far more difference from REPLACING the old single pane window with a modern, low-E double-pane unit than from covering it with 2 additional layers of plastic film - but the cost would also be higher.