~50 lbs in 2 years?
No. That's juicing. With optimal diet, committed training, good genetics, just starting out...you could maybe gain ~20 lbs of muscle in a year, but that level of muscle gain just isn't happening year-to-year without a boost. People who boast of this type of crazy lean muscle gain (outside of puberty) are either,
1) Misinformed (ex. maybe they gained 20 lbs in a year, but far from it is all muscle...or, they're reading an inaccurate measurement)
2) Lying
3) Juicing