I'm graduating from Chemical Engineering this year but I want to work in tech start up scene. Should I pick up a double major in Computer Science? (Extra 2 years)

As a fellow ChE who made transition to tech, I will suggest not to pursue double major. Instead, find a job with process control, automation, simulation, instrumentation vendors serving the chemical industry. These jobs are heavily computer, programming, and development oriented. In couple of years you will be well versed in computer technology with added benefit of domain experience. Also, some companies will sponsor you to get masters degree in computer science part time as they find easier to train ChE to do computer science than teach CompSci about chemical industry. If you can get a job with advanced process control (APC) vendor, you will be in much more demand later on with analytics, web and data oriented companies that are just dipping their toes in neural networks and machine learning techniques that APC has been using for 20 years.

Considering that you are from chemical engineering background, it's better that you join a startup right after graduation. You already have all the analytical skills need to be a good programmer. If I am not wrong you might have already done some programming as part of your engineering curriculum. So, you already have the required skills needed. A couple of years working on real world problems will give you more experience than studying computer science in a class room environment. Working part-time has some cons. You won't be able to cope with all the exams that will go on and this might affect your performance. IMO, this would depend on your ability to handle multiple things a time. Also, as you want to join a tech startup after double major, this might be a better choice. That said, it is very essential that you find a startup which has experienced programmers working in it and are ready to invest time in you. I wouldn't really recommend diving in when you don't have a proper mentor at the startup.

When early-stage startups make hires, it's typically based not on degrees or credentials, but based on projects or accomplishments. You won't get a better education by doing it in school rather than hacking on projects, and if you do projects you'll be able to show relevant, cool things. This helps the startup feel confident that you can and do build things, not that you know how to build things. If you're looking at a later-stage startup (think Dropbox or Square, or as another answer pointed out, Google) they are more likely to value formal education as well as what you've done. The projects you work on will be more specialized, and the academic knowledge will be more important for optimizing things, improving speed and performance, building it "the right way" rather than just building something that works. So in that case, it might be worth getting the degree.

Do the double major while working on startup projects and building a team on the side.

Take the time to go back to school. This is true especially if you want to work at companies like Google who expect their engineers to understand how to actually program, not hack code. You will be glad you did. Besides, there's nothing wrong with taking all of your alternate options at the same time. This way you will be well rounded.

If you really want to work in the start up scene, forget the school, find a team and product you completely believe in and add as much value as you possibily can.