I could be wrong but I was under the impression that the use of multiple NDB's,was to provide a further descent/descent limitation fix (no DME),rather than to aid tracking accuracy.Hence, both NDB's must be servicable but you can fly the proceedure with a single ADF, or with both ADF's tuned to the same aid
In the interest of orientation though I would have thought it more practable to allocate an ADF to each NDB as you described.
Remember the Westwind in Alice.