We periodically lose all water pressure in the building. I purchased a 100 gallon non pressurized water storage tank with a float valve and a pressure pump with 3 gallon pressure tank. The only thing using water is a kitchen sink with a commercial faucet that uses very little water. Assume building pressure is 55 PSI (when available) and I set the pressure pump to 50 PSI. All piping is 3/4".
The float valve will keep my tank full when water is available. My question is - do I need to manually control what supplies the sink, or can I allow both the normal supply and the pressure tank to feed the sink simultaneously? If there's building pressure, it will fill the pressure tank to 55 PSI, so the pump won't start. However, when the sink starts to draw water and the pressure drops, I don't want the pump to start and become the prime supplier of water from the tank.

Your description of the system seems to indicate that because it is pressure induced, the secondary pump will only activate during a drop in pressure. Otherwise, it is always on standby when the building pressure is normal.

Yes you do need to manually control it. The other problem you will have is that when you lose pressure, your pressure pump will try to back pressurize the entire incoming water supply and mains.

There is a way to automatically rig it, but you'll need some additional hardware.

You need a pressure switch and a solenoid/ electric valve.

The pressure switch will be positioned on the mains so that when pressure drops below a set level, the electric valve will divert your building's pipes from being supplied by the incoming city water to your own internal pump and pressure tank. When the city water comes back on, the switch will divert back to being supplied by the mains.

You can do the same basic thing with a check valve installed on the main incoming line and a T where the line goes to the holding tank. check the line coming off the T and attach it to the output of the holding tank feeding the building if I understand the setup correctly. Specific requirements may vary, but while there is pressure, the pump won't operate, the holding tank will be full and inlet shut off with the float. Water will bypass around the holding tank via. the T in the line and supply the building. In the case of interrupted water supply, the incoming main will lose pressure and the pump will activate, but a check on the line from the T to the building distribution will prevent the water from cycling from the pump to the fill on the tank and maintain pressure until the building supply is restored and the holding tank refilled. Simple, automatic, and doesn't rely on solenoids to operate or additional pressure switches.