Would you say the Octatrack has the functionality of a small mixer but not so direct I guess?
I only know less to nothing about the device and has enough content at the moment with the AR and A4.
But who know´s, the Digitakt and the Octatrack sounds interesting and why not in the future. 
OT can be used as a mixer / multi fx / recorder with A4 and AR, no problem. Up to 8 serial fx, fx send (CUE recording), Master track fx…
DT can record 30 sec, mono.
Besides this topic, is it possible to transfer samples to the rytm in stereo and use them stereo?
Or is only mono sample playback possible?
I only know that sampling with the ar is only possible in 1 line mono or 2 line mono.
Mono samples only, but you can use 2 mono samples and hard pan them to play them as a stereo sample.