How do I automate data transfer between two SharePoint lists?

Hello,

I will try to explain this as best as I can.

We have two sharepoint lists.

List 1 is the main list

List 2 is based on an email we sent out (mail merge), clicking the customized link in that survey will link them list 2 on the sharepoint, where they will create their own row in the list, filling out the survey.

List 1 and 2 both have a primary key (ID).  

I need to find a way to automate the process where the survey responses resulting from list 2 are displayed in the correspond row (based on primary key) in List 1.  Or ideally, just merge the two rows.

Did that make any sense?  How can I do this?  

Thanks!