What if we can choose the same mastery again, so if you are a soldier then at level 10 select soldier again as a second mastery - you get something like “super soldier” or “super demolition” type class. A grandmaster class in which your skills get ramp up by two. Is this even possible to mod?
You would need to create 12 masteries to do this, and the two variants would have to be slightly different and not reuse the same files because if they do you’d just have two masteries maxed out with the same skills and not double effects.
Wow, sounds complicated.
So basically you want to spend more levels in the same skill/mastery…
So why not add extra skill levels to the base mastery and add more level to the training bar? 
Way less complicated like that!
You can probably do it yourself if you read the forum. 