ANFI - Oh! the joy of your SEH/MEH debate. To be clear though, the EC135 has effectively 3 separate tanks. 2 transfers pumps (one effectively for redundancy) and no booster pumps. Its simple. Provided you understand how its supposed to work and be managed in flight. Almost identical to the BO105.
The helicopter I operate at the moment is about 40 years older by design, has 10 tanks, cross-feeds, interconnects, transfer pumps, booster pumps and a bunch of NRVs. The main reason for all this gubbins is the design of the airframe leaving only small spaces to house the fuel meaning lots of tanks.
In the case of the EC135, fuel starvation. The difference between the two supply tanks offers the pilots 30-40 seconds to react to what could happen next. Its not a lot and it requires the pilot to know this. Its the second engine that offers this chance.
Having said that, a pilot who understands the EC135 fuel system should NEVER end up with fuel starvation provided the contents and warning systems are working correctly!!! To put it another way, the opportunity for a pilot to remain in flight thinking "Ah, in a few moments the first engine will flame out and I will have 30-40 seconds to get the lever down before the second engine flames out" is/should be non-existent. What I mean by this, is if knowledge, understanding and comprehension is fully formed, a landing/ditching should have been carried out under power AND the pilot would NEVER try a relight knowing he has effectively run out of fuel.
So in this case, where a relight seems to have been attempted, two possibilities exist. either the pilot did not understand his system, indications etc OR the system//indications were misleading him to believe that the first engine flame out was not due to fuel starvation. Bearing in mind approx. 80kgs of fuel were remaining in the "system" albeit in the wrong place.