The training material at the link above seems to agree with ACCA Manual T in that although returns have little effect on room air circulation, they are best placed where the room stagnant zones are anticipated to be. For cooling, this would be near the ceiling, and for heating it would be near the floor. A high and low return system could accomplish this end, but my gut feeling is the return location should be based on the prevailing climate condition....a cooling or heating climate. Cooling climates would call for high returns, with heating climates having low returns.
In my neck of the woods the return location is determined mainly by equipment type. An upflow furnace/air handler on a slab foundation with ducts in the attic above or a soffit get low returns. Attic horizontal furnaces/air handlers get high returns.
For a dual return arrangement to be effective for most homeowners, if the extra cost could be justified, an automatic shift between high and low intake would be necessary. I'm just not sure it is worth the extra effort. Good supply air distribution overcomes a lot of thermal problems within a room, yet so many residential installations in my area use cheap stamped steel supplies either on the walls or ceilings (most common) and blast the air down into the occupied zone, leaving stagnant pockets everywhere else. Couple this with leaky thermal envelopes (walls, windows, doors, and ceilings) and marginal HVAC system installs, and is it any wonder we read posts such as the OP asking how to improve their comfort condition?