Well the problem with going that rought is you move the weak point further up the drive train.
With 4.10's in the diffs it requires more torqe to turn the tires over. When you double the cases you will encrease torque as your power moves from the first case to the second. If the torque needed to turn your pinion is higher than the braking point of the second case input shaft, it will break. This failure is most comon with the 21 spline cases but is still an issue with the 23 spline doublers.
Think of it this way...Remember back to your old 10 speed. If you put it in low gear you could pop wheeleis with ease, you barly had to put any effort into it. But if you put it into the highest gear it took all your weight just to push the pettle down. Now pretend you are 300 lbs, 90 years old, your front tire is against the curb and your in high gear. You go to put all your wight onto the pettel and your leg is going to brake. But if you were in low it would pop right over the curb with out much effort!
The best solution to all this is to buy a quality set of 5.29's and have them heat treated. Then do your doublers, and if you still feel you need more gearing through some 4.70's in the rear case. Another upgread to look at if you find yourself braking the front pinion is throughing a High pinion 3rd in it. That will increase the front R&P strength by 20-30% because you will be driving on the drive side of the gear rather than the coast side of the gear.
