I have a 95 corvette, does the secondary fan only come on with the A/C running?

Yes, the secondary fan is there specifically for extra cooling needs that can occur when you are running the ac. Especially on really hot days.

When you car reaches a set temp. the single fan comes on to cool the engine or keep it at the set temp for optimal running. If you activate the fan, additional load is placed on the engine so the second fan comes on to assist in the cooling of the engine. If you are not using the A/C and the engine is getting to hot for the one fan to cool it, at a preset temp. the second fan will activate and assist in the cooling. Gages are made different in analog and diigital. Analog uses a spring to activate the needle movement while the digital uses electronic milliamp flow to move the needle. One or the other may be off calibration. I'd go with the electronic digital as it has proven to be less variant in readings. If both fans are running and your engine is running hot, have the fluid level checked, and even burped in case there is air in the system. That also could give dual readings on your two gages. Next have the hoses checked for softness which could collapse when the engine gets hot and restrict the coolant flow. Is the antifreeze to water mixture correct? Sometimes when the coolant gets low only water is added and it becomes more water that a set mix, which will heat quicker than the correct mix. Next would be to again check the thermostat and make sure it is the correct temp. one and in correctly.