Matt, I would like to point out that I have seen a few Basses with 2 piece blocks across the width as well as Laminated Blocks in the depth. Also, a few of my Basses that were restored had an added piece laminated for the depth to either deepen the neck set and strengthen the bottom of the block or due to the block being cut from the top to shorten the bass.
My Hart had it's original Blocks inside it when it was opened up. The upper and lower blocks were two pieced Pine. It seems that this was either done due to a shortage of materials or perhaps a method for stability. We will never know the actual 'why' these things were done in the past but if they survived 180 years, I think they got it right.
In the case of this French Bass you are working on which looks to be a Mirecourt production rather then a makers individual single made bass, Blocks and other materials were used as they were supplied to the workers. I would assume that if the part was bad in the beginning, it would be tossed and not used but foresight in which what might survive the future or not was not a decision of a single worker.
Also, from the looks of the condition of this bass being left alone for so long, dryness and stress combined can easily split any wide straight grained piece of wood like this Block. It can also split from the other pieces of wood glued to it as the weakest link is what usually gives. So, if you think this Bass needs some extra strength in the block material itself, two pieces glued of slightly different grain might help. Just my 2 cents, or 3 or 4..
Keep up the good work. I bet you can't wait for the next big job to walk into your shop huh?..lol