I honestly don't know the answer to this one. It seems that the aft motor would benefit from the higher pitch but I don't know of anywhere I have seen a study performed on the requirements.

As a rough approach I would prop the first motor as you plan to do and take a tach reading. Then with the first motor running I would prop the second motor to give the same tach reading.

With the various models that the members of the forum have built (they after all old and seasoned modelers) I would think at least one would have first hand data.

Do you have a pict or drawing of what the boat will look like? I was spell bound by the first airboat that I saw as it flew over a mixture of mud and wet grass in Florida. I made one many years ago but at the speeds it went it was more of a barge than an airboat