There can only be one crusade active for one faction at a time.

So if they sent two crusades one soon after the other, you can bet the first one was a dismal failiure (ie had no retreat, or very few men and everyone wound up dead within a few turns)

The AI sucks at doing crusades because it is not smart enough to add units to it as it progresses, as such most of the AI crusades seem to be full of Peasants and Fanatics with a handful of Knights, Order Foot Soldiers and any other units that may have joined up along the way.

This is another reason why many Catholic factions implode with Civil War on the map, because the dramatic loss of influence brought about by a failed crusade, caused global loyalty drops... and makes civil war very likely.

Of course Muslim Jihads I find far more useful because they can be used against ANYONE (even other Muslims) and you can have more than one going at a time... the Downside is that you can only target provinces you once held, but this can work well if you attack a province you don't intend to hold with a small force, and then withdraw and call up a Jihad later... Suddenly you have a larger army, your kings influence and Piety is better, and you are in a much better strategic position.