In theory nothing prevents you from going single class. You max out your selected mastery bar and spend points into skills. However health does matter in later difficulties, and as a result of health shortage you will die.
Yes, you can go for secondary mastery and invest in mastery points only. In reality this is a dual class build, not pure single class build, and there are some differences. It’s safe to conclude that only dual class warriors are viable, thus being possible 15 combinations to make. Those differences are:
1.) class name
2.) attribute, health and energy points.
PASSIVE SKILLS AND EQUIPMENT ARE EXCLUDED FROM THIS CALCULATION
2.) With the current possibility of mastery points allocation Warder class has the most health - 4075.
With double points for single class Soldier would have 4550.
With current possibility of mastery points allocation Warlock class has the most energy - 3100.
With double points for single class Arcanist would have 3500.
The same applies for physique, cunning and spirit.
How to make this possible?
You are about to fill your mastery bar to the maximum: https://s11.postimg.org/v1wq0shw3/image.jpg
When you reach mastery bar cap, you get two options. Choose your secondary mastery, or start filling the same class mastery below the existing mastery bar: https://s22.postimg.org/a3tmp1235/image.jpg
Investing in the same mastery bar continues providing you with the same amount of attributes, and will disable your secondary mastery selection, but will offer you to undo current class selection, since you became pure Soldier class: https://s10.postimg.org/ugrd4fmjt/image.jpg
Pros
We get additional 6 viable classes, 21 in total, instead of current 15.
Crate Entertainment gets to brag with 21 possible classes, plus later imagine that number of classes with the upcoming Inquisitor class and another one.
Cons
Crate Entertainment get more shit to do.