Flixotide is a steroid inhaler (preventer) and should be used as prescribed once or twice a day (usually morning and night). Ventolin is a reliever and used to help during an attack of asthma. They can be used at the same time if an asthma attack occurs at the time the preventer is being administered.
The reliever (ventolin/salbutamol) should be used as soon as you notice an attack occurring, don't leave it too long as it may not work.
I've had asthma myself for over 20 years so am very familiar with the different inhalers and routines :-)