The problem with your example is requirements are not that strict. You don't need to move data from DB1 -> DB2 within a single clock cycle. And if you did som

The problem with your example is requirements are not that strict. You don't need to move data from DB1 -> DB2 within a single clock cycle. And if you did some systems would think they saw the same data in both systems due to latency and caching etc. Thus the compromise of using a hidden intermediary (or blocking DB access) is not only reasonable but necessarily. For stuff like this the absolute worst case is you update one or more legacy systems. But, that's just part of the cost / benefit / risk analysis.