Hi zizi! Good to hear you l’ve also got your hands on this awesome setup. I’m still madly in love with both these machines after all this time, both feel like timeless instruments. I think there’s multiple ways in which to connect the 2, so theres no wrong or right way.
My setup is pretty simple: the digitakt has a midi out cable going in to the grandmother, and the grandmother can be routed to the digitakt input via my mixer. I use the first midi channel on the dt to control/sequence the grandmother, and if i got something i like, i record it into the dt and put it on one of the audio channels, and move onto the next sound on the GM, by twisting knobs until i have another cool sound.
Theres a lot of other ways to use them together though, i think you could use the GM as the controller keyboard for the DT as well as a sound source by connecting midi in/out on both decices using 2 cables, but you’ll have to turn of local control on the GM to avoid a midi loop (i think this is possible, you should check the manual). You could also try sending clock to the GM to control its sequencer or arp, but i’m not sure how this works as i only use the DT sequencer. Good luck, whatever way you’ll get them to work together!