You'll want to set a variable that prevents the latter trigger from being triggered until the former trigger has completed. In Prototype you could do something like the following:
Code:
var bTrigger = true;
if(bTrigger == true)
{
bTrigger = false;
pEvent =
{
afterFinish: function()
{
bTrigger = true;
}
}
new Effect.Shake($('myElement'), pEvent);
}