Hi Subin,
The first approach is a lot easier to manage and involves lesser risks.
Also as Vag rightly points that in case of different Vendors co-ordination is also an important point to consider before designing such a landscape.
We too used the first approach and once our major change was done,we merged the systems which in your case are DEV2 and QA2.This was a much cleaner approach.
Thanks,
Ravi