Timestamp consistency of dependent system-version tables

I want to system-version some items that consists of data in forms of blobs as well as metadata. I plan to use two tables, say Metadata and Data, to store them. Data has a metadata_id to refer to the metadata. (It could be the other way round actually, I am not sure which way around is more efficient in practice). I want to make sure that when I query the history of the DB, I get a consistent DB. However that is not the case, because the timestamps that are present in Data and Metadata are not consistent, even when they are jointly filled within a transaction. (Sure, the time delta might be small, but I am looking for guaranteed perfect equality here). How is this situation handled in practice? Should system versioning be used only on tables that do not need to be filled in a transaction along with other tables? Should I put my entries in a single row despite having large blobs? Should I implement my own versioning system?