The two tubes need to be wired in series (otherwise you will get one lamp on and not the other) and it depends on what the striking voltage and current rating of the ballast is. If the striking voltage is high enough, then the two lamps will light. You need to make sure that the total wattage of the lamps is about the same as the rating of the ballast too, otherwise it may overheat.
Generally, it is best to use the ballast in the configuration it was designed for. If you do get two lamps to strike, leave them on for a while and monitor the ballast carefully; fire is always a risk if the ballast is being overloaded.