Hi JDM1052 I think the easiest way to do this would be to just create one survey with all your pre and post questions and enter both of the responses from each individual in a single survey response. This way, they will be paired together and you won't have to link across surveys. Would that work for you?

Unfortunately, the post-survey isn't done yet. It will be coming up in a few months. I have to analyze data from the pre-survey and it is easier to use the Qualtrics SPSS output than to build my own SPSS from scratch. So for ease of the future surveys I figured I would set the web version up to be longitudinal from the start. Otherwise, your idea is a great one.

Ah, I see. Then, is there anything keeping you from just adding a field to your survey where you enter the identifying information? I'm assuming you have a name, ID, or something like that. I would just set up a field for this purpose and then join the data files in SPSS using that field.
You could also theoretically still use a single survey, though it's a little more work-intensive. You'd program the survey to contain both the pre- and post-survey questions, including identifying information. Enter the pre-survey data for all participants. Then, when you have post-survey data, you can use the re-take survey link: https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/retake-survey-link/
The link retains the originally entered information, so you can just click through those pages and then enter the post-survey data once you get to those questions.