Carolus Magnus has PM me with this info:

Multiple crusades are indeed possible. If the Pope has been "discomfited mightily" by a faction and calls for a crusade, anyone may launch a free crusade against that faction during that turn, even if they already have an active crusade.

I've had three crusades active at once due to this, against three different factions. I don't know if multiple crusades may be started against one faction when the Pope calls for a crusade, I haven't found myself in a position to check this.

It seems that the Crusades work correctly

@Pericles and Weebeast
I don't think adding more money to the campagn in the mod files is a good idea (adding it to official mod), because the player will also start with a high amount of money. Most players of the mod properly can't mod or change the file on their own, so it properly best keep the low start money. Experiensed modders can make the changes, so that will not be a problem.

@Weebeast
I think there can be more than one jihad at the time, but im not shure. jihads doesn't work the same way as crusades, so it could be possible (have not tested it).