Paralleling a 12V battery with just one of the two series-connected 12V batteries in the bus might have started the bus if only one of the bus batteries was completely discharged or open. and the other still had sufficient charge (mismatched batteries). You might have got lucky and jumpered your good battery across the bad bus battery on the first try, but if the first try wasn't sucessful, then move the cables to the second battery.
There are many failure modes of lead-acid batteries that might kill one battery, but leave the other still capable of providing cranking current. For example, a dead cell, a shorted cell, an open plate jumper. When this happens to one battery in a series 24V system, both batteries should be replaced with new, because it is important that both batteries have the same capacity. The remaining good battery can be used up in a 12V car.