Does IIIT Lucknow get good placement because of IIIT Allahabad?

This is true that the only reason IIIT Lucknow has great placements is simply because of IIIT Allahabad. It can be explained with the help of the following points: 

1. The students of IIIT Lucknow share the same classrooms where they are taught by the same professors that impart the same skills to students of both institutes. 
2. The competition at IIIT Allahabad is intense and it includes one of the most brilliant brains in the whole of India. 
3. The companies that visit IIIT Allahabad are the same companies that visit IIIT Lucknow as well to select students as their future employees.

If you are concerned about the fact that it would affect IIIT Lucknow once they shift, the straight answer to this is YES! With this, it is going to get difficult to expect companies to show up.