What are the advantages and dis advantages of doing dual degree that means doing b.tech and m.tech in 5 years

Only benefit is saving on a year. Fee wise there wouldn't be much savings. Disadvantages are more because industry doesn't appreciate short cuts here unless & otherwise you are doing integrated dual degree from reputed NITs or IITs. Wish you all the very best.

In dual degree you save one year and so the fee is also saved besides time. Check it from some good college sites for such academic details: SRM, Lovely Professional Univ etc