I did my b.com in 2012 from purvanchal university now i have taken admission in b.sc at mumbai university on the basis of my 12 science

But why did you change. once a graduate in Commerce, you could have tried CAT/ MBA course with similar efforts as you will be required fro science.

Both are legal degree. You can pursue multiple degrees and all of them stands legal if you have done it through UGC approved university as per the UGC guideline subject to approval from other bodies