Question which is better to use as a subfloor on concrete floor in a high rise condo...

Two 3/8" boards crisscrossed and screwed together or one 3/4" plywood screwed to the concrete.?

I will be placing a 1/4" glued cork underlayment below the ply wood and place a 3/4" solid harwood on the top of the plywood> Thanks

Quite a few condo boards wont even allow you to screw into the concrete. This negates the effectiveness of the cork as a sound barrier.
If you use two layers of 3/8th inch plywood, you will have to use shorter fasteners for the hardwood to avoid penetrating too deeply.
Have you considered engineered hardwood to avoid the need for the plywood altogether?
Sometimes adding 3/4 ply plus 3/4 floor can create awkward transitions.
The engineered floors are generally 3/8 to 3/4 inch and can be glued directly to your cork. (dont go with the cheap types, remembering you are saving the cost of your plywood and consequent labour, so throw some of that saving into buying the finest engineered floor you can find.)

The floor is uneven in spots and I thought the solid harwwod would be a more solid walking surface then engineered floor that can give you a squishy feeling.I understand about losing the sound deeadening of the cork. I understand that trying to get a flat foor is the best but the existing floor has already has a glued down 1/8" asbetos plastic linoleum tile that I would want to remove...if possible
I want to use either two screwed together 3/8 plywood sheets floated on top of the cork ( not screwed to the concrete) or one screwed to the concrete 3/4 inch board. Then I would float the solid 3/4" on top of that ...which is best?

Two sheets of 3/8th inch is better, but why remove the tiles?
Anyway, you should still put some effort into flattening out the floor first before doing anything, especially if you float the subfloor system.