MBD does cause rubbery bones in legs which could explain why his leg is twisted, However, the fact it had orangy crusts stuck on it can indicate something else may be wrong. Unless the bearded dragons were kept on orangy calci-sand at the petstore, and because the dragon was having problems with his legs, the sand may have stuck on it from fluid when he did a bowel movement, ran across a dish of water or damp greens etc. It could also be discharge from an infection if the leg was in fact broken.
Taking both dragons to a vet for a checkup and examination would be the best thing to do. They can x-ray the dragon with the bent leg and see if it was caused by a break or MBD or something else. They can also do blood work and see what the blood calcium levels are, which would indicate they are getting enough calcium in their diet and enough UVB exposure to properly utilize the calcium they injest.
There are really three components bearded dragons need to have enough calcium in the body. A source of UVB (either natural u nfiltered sunlight, or provided by quality UVB producing lights, such as repti-glo, repti-sun 5.0's or higher, or mercury vapour bulbs. These bulbs will need to be changed every 6-9 months (even the mercury vapour bulbs should be changed at least once a year, even if the visible light is still strong). The reason why bearded dragons and other diurnal dragons (active during the day) need access to sufficient levels of UVB rays is so their bodes are able to produce Vitamine D3. Though suppliments can put Vitamine D3 into the bodies, it is very difficult to determine just how much D3 dragons need. Too much D3 is just as bad as too little. IT is much easier and more natural for dragons to regulate their own exposure to UVB and let their bodies manage how much D3 to produce. When th ey are taking it into their bodies via diet, they can't stop producing it as it's already in there and this can lead to toxic levels of D3 in the body.
What D3 is used for in the bearded dragon's body, is to take the calcium injested and utilize it for various body functions, not just formation of bones or egg shells (for pregnant females of course). Calcium is used in muscles to aid in transmission of signals via nerves etc...which is why dragons lacking in calcium either due to improper diet or insufficent UVB explosure, often develop twitching in the limbs.
Thats pretty much the simple explaination why Calcium, UVB and Vitamine D3 is important to a bearded dragon. Providing lightly dusted crickets (or other suitable insects) with calcium d3 and a multivitamine once a day (for dragons under 4 months old, dust one feeding of crickets once a day with calcium and vitamines, 4 month old dragons or younger should have 2-3 feedings of crickets every day). Above 4months up to about 8 months, dust once every other day but continue with 1-2 feedings of crickets a day, salad offered daily as well. Above 8 months, feed salad every day, crickets every other day, dust insects each offering (once every other day...). Adult bearded dragons, pretty much anything over 14 months, insects 2-3 times a week, salad every day, dust insects 1-2 times a week.
