[bob1029]

Insulation is way more important than cooling capacity.

Instead of adding a 2nd condensing unit, try adding spray foam and double glazed windows. I can keep my 2400 sqft Texas home ice cold with a small 3 ton system. There are similarly sized homes right down the street from me with 5+ tons of capacity across multiple condensing units.

The biggest problem with oversized cooling systems is latent heat removal. It's much better to have a slightly undersized system running constantly through the hottest part of the day than it is to have a very powerful system running with 50% duty cycle. It's also significantly better for system life (capacitors last much longer this way). During summer months, my hvac system cycles maybe one time between the hours of 10am and 10pm.