first question you have to ask yourself is, how do you want to match the data later on.
If you want to use an anonymous survey, you could give out the incrementing participant-number via #lfdn# and have your participant note it, but I think this leaves room for different problems.
Alternatively, you could ask for your participant's email adress at the end of your first survey and then use a mail trigger which sends the link of your second survey out x days later. There you can also include the aforementioned #lfdn# like this:
Please check back on the manual's section on URL parameters for further details.
If you prefer a personalized survey, you could set up two areas within one survey.
Then you could, could set up two filterpaths, both with an ending page. At the end of the first part, you would use a recoding trigger to set - let's say for explanation reasons - c_0001 = 1
The first filterpath checks on c_0001 = 0 and the second path c_0001 = 1
When you send out another invite x days later, anyone who already answered would be led into the second path.
Or you could use a mail trigger at the end of the first part, which sends out an invitational mail x days later, including a URL parameter which leads into the second part of the survey.
As you see, there are many different viable options.