ODK-X is a different project, so the XLSForms you've developed for ODK will need modification if you transfer over to those tools. ODK-X could likely do what you are looking for, but will probably require building the various pieces. You can always explore more on their forum and docs.
ODK has support for external form datasets in the format of CSV or XML. However, this functionality works better if the the collection forms are used in sequential stages - rather than simultaneously - as the external form dataset must be both added to the server and provisioned to the phones.
ODK Central is designed to make it easy to connect to other tools for data processing.
For example, via @GetODK on Twitter: "Google Data Studio (GDS) is a free data visualization tool, and thanks to UW Impact++, you can now use GDS to build interactive dashboards and beautiful reports of your ODK Central data. Try it at https://datastudio.google.com/data?search=ODK"
If you create "a simple web app or a script to retrieve all submission from both forms through the API and generate a brief report on the comparison" we would love to hear about the experience and how it goes. It sounds like you know about the OData connector which allows you connect a live representation of the data to OData-capable tools like Microsoft Excel, Microsoft Power BI, Tableau, SAP, and others. And that you've looked at the Central API documentation. Central is designed to facilitate setting up external analysis like you describe.
When thinking about your data flows, these other forum threads may be of interest to you: