I don't believe there is any setting (registry/ini) that can specify the order from ProcessBook.
I have read somewhere that addins for Microsoft applications are loaded in reverse alphabetical order, maybe ProcessBook inherits this too? (Haven't tested this)
Failing all that an unreliable method () you could adopt is to have a loading addin that has its startup behaviour set to loaded, whilst your other addins set to not loaded. Then in the loading addin you specify & control the order in which other addins are loaded/started. Problem with this being a user could change the startup behaviour...
The add-in loading is actually handled by VBA not ProcessBook, so it's whatever order Microsoft decides (and I don't know off hand what order that ends up being). One possible thing to try would be to make Add-In B have a reference to Add-In A. Hopefully this would cause Add-In A to load first. That assumes that VBA is smart enough to negotiate the load order based on what refers to what and that having Add-Ins reference each other isn't inherently bad (I've never tried).