I will share here my answer from the support ticket you sent us on the same topic, maybe it will be helpful to someone else as well:
The main problem in such cases is that currently a property of a persistent class can be mapped to only one column definition. Having this in mind, if the columns mapped to this property from the different backends differ in any way (different name, type or length), we will not be able to store the column information specific for each backend but for only one of them. We have planned to avoid this limitation by providing functionality to store database specific information for multiple backends in one model but it is not guaranteed that this will happen for the upcoming Q1 release.
However, if you use entirely forward mapping, without specifying any relational parts in the model (tables/columns) and let OpenAccess use its default mapping mechanism, the model would work with any backend. The drawback is that you will not have control over the table definitions as they have to be produced by OpenAccess during runtime.
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