Tom,
I doubt any work would require you to have dual drains on old pools... You can just get the VGBA cover...
I would either just install the correct cover on the single drain you have, or abandon the main drain all together, as they are just not needed.
I have two rent houses that were re-plastered.. In both cases the plaster company initially said they were "required".. I asked to see the requirement and then they said it was "recommended" and they just replaced the flat cover with a raised cover. At that time I did not realize how useless a main drain really is, or I would have just told them to cap it off and plaster over it.
Thanks,
Jim R.