When you complete a course of four years normally, you complete your graduation with the fundamentals of your branch which allows you to pursue higher studies if they are in your interests. It also gives you the option, at the same time, to complete your undergraduate degree in another subject if you wish to.
If you want to pursue a dual course, it can probably put a dent in your freedom to pursue your interests and you will be ‘rewarded’ with the master’s degree. Another problem after receiving the master’s degree would be the confusion about looking for a job, pursuing a doctorate in the field of your interest(s).
The dual degree in mechanical will demand more dedication from you but the four-year degrees are easier compared to those. Mechanical engineering is a good option if going for higher studies is in your interests.