To charge a system with the engine running, the refrigerant should be added to the low side of the system.
Never open the high-side hand valve with the system operating and a refrigerant can connected to the center hose. The refrigerant will flow out of the system under high pressure in to the can. High-side pressure is between 150 and 300 psi and will cause the refrigerant tank to burst.
Recovery, recycling, reclaims.
The only occasion for opening both hand valves at the same time would be when evacuating the system or when recovering the refrigerant with the system off.