Dear Garima, 

Yes, you are allowed to apply to a maximum of two disciplines for NID DAT. In your case since you wish to apply for both Information Design and Interaction Design, you can fill those. Since, both the disciplines fall under one faculty, you will be required to appear for one domain specific test only. The prospects say: 

For those applying for two disciplines of M.Des., they will be required to appear for: "The CDAT [Common Design Aptitude Test] and ONE domain-specific test if the two disciplines selected falls under same Faculty Stream. The total duration of DAT Prelim will be 2.5 hrs."

Coming to whether you can edit your choices, you may or may not be able to add more choices when the opportunity to edit the application form arise, since certain fields cannot be edited such as DOB, email ID, Category and citizenship cannot be edited. You can try your luck during this editing window. The window to edit the application forms will start from 12 noon on Saturday, 1 December 2018, and will remain open until 12 noon on Tuesday, 4 December 2018. 

With respect to disciplines, the brochure says, "the discipline applied for CANNOT be modified." While you will not be able to modify it might be possible for you to ADD another discipline. 

I also suggest you to write to NID admission cell in the meanwhile. You can send an email to them at admissions@nid.edu

All the best!