Can I use two layers of window film to create a narrow, convection-reducing gap?
Reducing convection is a minor concern. The benefit described in that answer is trivial compared to the benefit of sealing against airflow through leaky windows. As a proportion of total heat loss, convection is very small as compared to drafts.

There would be a benefit to adding a second layer, but not primarily due to convection restriction.

A second layer of film adds insulation on its own. I wouldn't clutter your view with bubble wrap, which provides relatively little additional benefit. Because the seal between the first and second layer of film is (presumably) better than the seal between the inner layer and the outside world (due to a leaky window), the net benefit of a second layer is already better than the benefit from just the first layer.


All that said, the savings afforded by a second layer may not outweigh the cost of the product. My advice (having owned older homes in Minnesota) is to do a good job of installing one layer and be happy.