Several aircraft have been built with both a pull (or tractor)
propeller and a push (or rear) propeller aligned with each other.

Supposedly the arrangment is inefficient, yet the some of the Dornier
aircraft were record breakers.

Given that some aircraft were ruined or delayed by the problem of
combining gearboxes (He 177, Northrop XB-35 and if it ever came to it
the B-29 with its backup V-3420) it looks like an attractive
proposition.

What's going on here?

List of such aircraft:

Cessna 337 Skymaster
Rutan Model 76 Vogager
Adam A500

Dornier Wal
Dornier Do X
Dornier Do 18 Seaplane, the German PPY Catalina.
Dornier Do 26K Seaplane, possibly the longer ranged seaplane ever
built.
Dornier Do 335 Pfeil (arrow) perhaps one of the fastest piston
engined aircraft ever built.
Dornier Seawings Seastar, modern Seaplane of composites.

Savoia-Marchetti S.55
LeO H-242