I have also seen this done a few times. I don’t know of any statutory prohibition, but I have to say that filing a joint succession is just bad practice.
For one thing, the clerk of court is just going to have to split the filing into two cases to be able to index things properly. If the idea is to save court costs, I doubt if it really helps that much.
Another problem could be with the title to property. I can easily see a title examiner getting a bit nervous when it comes to a pending sale. Normally you have a succession for the first to die of a married couple that lists one-half the value of property since it is community. Assuming there is a will that leaves everything to the survivor, then when the second spouse dies there is another succession listing everything at full value. I doubt if any of this could be made clear if you tried to put it all together in one filing.