Well, in my humble opinion I think that it is possible for a 2nd action potential to be generated down axon whilst the initial one is still travelling. This depends on the strength of the stimulus and the state of the ion channels in the axon's membrane. 

Immediately following depolarization during action potential propagation the membrane enters into what is known as a refractory period, whereby potassium channels remain open and sodium channels are fixed in the closed position. This makes it very difficult to depolarize that portion of the membrane. 

Additionally, there are 2 different refractory periods, the absolute and the relative. The absolute refractory period lasts for almost the entire duration of the action potential within that section whereas the relative refractory period occurs directly after. It is during the relative refractory period that an action potential could be generated if the stimulus is strong enough. 

Thus, if the axon is long, by the time the action potential has reached the terminal button/end of the axon, the proximal region of the axon may have already entered the relative refractory period and could potentially undergo another depolarization...

Hope this helps.....

Cheers, 

Jake