We have an application utilising 27 implementation layers. We tried putting each implemetation layers work/work_history tables in separate database schemas using different Datasources to address these tables. Worked a charm until we tried upgrading (Pega 7.2.0 to 7.2.1) where the upgrade process crashed due to more than one data schema. Now we have a business case where we need to constantly cull (resolved) data from the work tables into separate archive db. This calls for using alternative datasources again. The question in the SR was if you now support using multiple datasources also when upgrading. apparently not. We find this strange that you support it when running the application but not in the upgrade process. Therefore we request that you amend the upgrade process to support usage of multiple datasources.
We use the Installation and Upgrade Assistant (IUA) tool when upgrading
The multipe datasource functionality which you have described in this post is expected to work. And your description says that it does, except when you attempted an upgrade. This implies there there is an issue with the upgrade scripts or process that needs to be looked into.
In that light, we suggest that you proceed to modify your application to use multiple data sources, attempt an upgrade and if it does not work, open an SR to pursue the issue.
You may want to start with two data sources as a simple test to see if the upgrade completes successfully.
While this testing is at your convenience, I request that you start with sufficient time for the GCS team to investigate and resolve any issues before those issues become blockers for your upgrade project.