Which field has more scope among BCA and engineering?

If you are talking about CS in both then BCA is much better than engineering but apart from CS branch engineering is better than doing BCA.

BCA + MCA = B. TECH B. TECH have a large variety of technical knowledge

Definitely both hve scopes but don't think about that think what you wanted to do. If there is interest in what r u doing than defenetely you should be successfully.

B.tech is an engineering degree of four years in which your two years course is same like for all other branches and 3rd year starts with specialization i.e theory of IT with practice and it includes the topic done in BCA whereas BCA is a generic degree of 3 years mainly includes subjects involving