(Bloomberg Opinion) -- If you’re looking for one of the worst ideas in contemporary banking, look no further than Germany.

The mooted merger between Deutsche Bank AG and Commerzbank AG would make a mockery of any notion that EU governments are serious about ending the “too big to fail” problem. It would also turn back the clock on a guiding principle of European regulation over the past decade: The promotion of a “banking union,” where risks are shared widely across the continent on the basis of jointly decided rules.

Berlin clearly has a problem when it comes to Deutsche, its most iconic lender. The bank has come under the scrutiny of regulators across the world because of its large portfolio of illiquid assets, as well as its implication in a string of money-laundering scandals and its dubious and relatively weak capital position. A succession of chief executives has failed to shrink costs sufficiently and revamp revenues. The share price has plummeted, leaving German politicians scrambling for a fix.

One proposed solution is to tie up Deutsche with Commerzbank, a rival lender that has already benefited from a cash injection from the state. The imagined benefits include: Cutting costs by reducing overlaps; hiding Deutsche Bank’s shaky balance sheet in a bigger entity; and creating a “national champion,” which can support German companies across the world. A merger might also spare the German government the embarrassment of having to consider an injection of public money. This would cause a domestic political backlash, and accusations of hypocrisy from abroad. It was Berlin, after all, that asked for stricter European rules on bank bailouts.

It’s not as if any of those supposed advantages of a merger stand up to scrutiny. The two banks specialize in different areas (Deutsche in investment banking, Commerzbank in trade financing), weakening the case for finding efficiency gains and savings. Meanwhile, combining the two banks would create an entity with more than $2 trillion in assets, mostly concentrated in a single country rather than spread across national borders. Finally, any politician who thinks there’s a need for a national champion should look no further than Deutsche Bank itself, and see how well that went.

Would European supervisors even be able to supervise such a behemoth? Perhaps. But it’s far less certain that they’d be able to close it down in an orderly manner, if that was ever needed. The Single Resolution Board, the European agency in charge of winding down failing banks, is currently working with lenders on drafting plans so that such an event could be managed over a single weekend. Deutsche Bank is already the name that pops up every time someone questions the resolution board’s ability to cope with a complex bank that might get into trouble. Adding Commerzbank would merely make this worse.