Which cases are there of journeys where you could reasonably take two back-to-back night trains (which I'm defining as "trains with couchettes / beds", not the 24-hour Thameslink services) in which that is not the only possibility?
For example, I wouldn't count Cerbère – Paris – Nice as there's daytime trains which get you there quicker than Intercités de Nuit to Paris and back. I wouldn't count Amsterdam – Prague – Warsaw as you could get the European Sleeper then daytime trains, changing at Ostrava, to Warsaw and arrive in Warsaw half an hour before the sleeper leaves Prague. To make things more interesting, I'd also exclude any stations / routes only served by overnight trains, so e.g. no Topeka – Chicago – Omaha.
One that seems to fit is Brussels – Passau. Looking at November due to ÖBB giving up on Brussels due to track work until then, the Nightjet to Vienna leaves Midi at 18:33 and gets to Köln Hbf at 21:50, with the Amsterdam – Vienna Nightjet leaving at 22:27 and getting to Passau at 06:13. It's not a perfect example, as the Brussels – Köln is quicker enough with the Eurostar that journey planners suggest the 19:25 departure, but it is a case otherwise fitting the criteria.
For example, I wouldn't count Cerbère – Paris – Nice as there's daytime trains which get you there quicker than Intercités de Nuit to Paris and back. I wouldn't count Amsterdam – Prague – Warsaw as you could get the European Sleeper then daytime trains, changing at Ostrava, to Warsaw and arrive in Warsaw half an hour before the sleeper leaves Prague. To make things more interesting, I'd also exclude any stations / routes only served by overnight trains, so e.g. no Topeka – Chicago – Omaha.
One that seems to fit is Brussels – Passau. Looking at November due to ÖBB giving up on Brussels due to track work until then, the Nightjet to Vienna leaves Midi at 18:33 and gets to Köln Hbf at 21:50, with the Amsterdam – Vienna Nightjet leaving at 22:27 and getting to Passau at 06:13. It's not a perfect example, as the Brussels – Köln is quicker enough with the Eurostar that journey planners suggest the 19:25 departure, but it is a case otherwise fitting the criteria.