Does our main air intake need an electrostatic filter if our furnace already has one?
Question of whether you can do it depends on the static pressure load your furnace is already dealing with.  The nameplate on your furnace will specify the maximum static pressure allowance.  This is basically a measure of how much resistance the furnace blower feels when pushing air.

Whatever you do in whatever configuration of filters including allowances for dirty filters you need to stay below this limit at all times to prevent potentially serious problems including cracked heat exchangers and frosting over of AC coils.  The fancier pleated/electrostatic filters with high ratings for dust collection significantly increase overall static pressure v. cheap fiberglass.

If the guy has no clue what the static pressure of the system is and did not have a manometer to check it with both filters installed I wouldn't go there.  I know my system couldn't handle it.

On question of should you.. I suppose having filters would protect return ducts from new dust not already filtered out of the air by main filter.