You can concatenate the required values or just use random number generator in survey flow to get the desired combination for your result.

Thanks for your input! I see how I can use the random number generator in survey flow, but I need the same random number to be assigned to the respondent in the pre and post survey. So I don't see how the random number generator can do that. The concatenate values is interesting, but again I don't see how I can use that to assign the same number to a respondent on the pre and post.
It will be mostly the same respondents in the pre and post survey, but some people may never take the post or some may not take the pre but they do take the post.

Hi,

I have a help sheet on how to do this at https://researchguides.library.syr.edu/dataservices/Qualtrics . Scroll down to Special Topics and look for the Repeated Measures in Qualtrics doc.

I really like your website and clear instructions! Also the accessibility guide is great.
So I have my pre/post survey set up so that it is one survey with two branches (pre/post), and this way I can create live reports that compare data as students fill out the survey. From what I understand, you can't create a report in qualtrics that pulls from two separate surveys. 
Also, now I have one link set up that different university reps send to their students. It would be very difficult for me to set up email tasks for the post survey because students are completing their course and post survey at many different times throughout the year, and thousands of students are taking the survey. 
So in my case, it might not be possible for qualtrics to use the name or email to generate the same uniqueid for students on the pre/post survey. I guess there are trade offs with the different features. However, if you have any other ideas, I am all ears!