Is a configuration with two separate reverse osmosis stages, where the permeate from the first stage is fed into the second stage, an example of a double-pass RO unit?

Yes, a double-pass RO unit operates by passing the permeate from the first stage into a second stage, allowing for further removal of dissolved solids and other contaminants, which is particularly useful for brackish or high-salinity water sources.

Source: Membrane system design and operation references