No point in doing Btech after Bsc.

Either you go for Msc or you take up Btech first an dthen go for Mtech in Mechanical engineering.

Both streams have different course curriculum and hence coukd be difficult.
Overall its wastage of 4years post Bsc.

You should take up Btech first in Mechanical engineering and probably then go for Mtech which is best possible combination interms of ROI.