You would need a USB MIDI Host device. You could plug both the DT and the MPK Mini into it. If you have a Raspberry Pi kicking around you can make one in less than 10 minutes:
I bought both the Akai MPK Mini MKIII and the Novation Launchkey Mini MKIII to use with the Deluge via USB host. Both sent MIDI via USB and received it as well over USB (which was good for syncing their internal arpeggiators to the clock). I kept the Novation. The Akai had one thing that I miss… the note repeat for the pads. Other than that, the Novation won out on many levels. Keys played nicer. 16 Pads that feel pretty decent, especially for light touch playing. A more interesting arp. Better pitch bend and mod wheel control. And… a MIDI out jack. It works great for travel with the Deluge, but spends most of its time sending MIDI notes to gear like my NTS-1 and even my Tempest.
Yes. You’ll need to power it with a USB power plug. Also, it only has MIDI Out via a TRS jack, no MIDI IN, so it will not be able to receive clock externally for the Arp, which is the only real bummer. Wish it had TRS MIDI In. Would love to find out there’s a mod for that.
Depending on what exactly you want the controller for (knobs & pads to control parameters remotely vs keys to play the DT with), I‘d recommend you go the Keystep route if you‘re aim is primarily to have keys to play your music.
The Keystep has a simple sequencer, a good arpeggiator, and it has midi in/out which means you don‘t need any adaptors and you can sync the keystep to your Digitakt‘s clock (which makes the arpeggiator play in time with your Digitakt).
Plus the Keystep allows for on-the-fly switching of the midi channel it sends to, which gives you a little more flexibility in addressing your gear.
The Launchkey Mini Mk3 is a sweet little controller as well of course, but the fact that it doesn‘t have a midi input to receive clock to me makes it a no go for my hardware setup.
I should add that I own both, a Launchkey Mini Mk3 and an OG Arturia Keystep. I use my Keystep with my hardware and the Launchkey Mini Mk3 with my laptop and Bitwig/Studio One when I travel.
Good advice here from @hausland. Launchkey Mini MK3 does allow you to change MIDI channels on the fly (with a key combo all 16 pads light up and you pick your channel). This affects the output of everything… the keys, pads, and knobs. There is some amount of configuration you can do with their app… assigning CCs for the knobs and Notes for the pads. Unfortunately, it does not have MIDI IN like the Keystep, so yes… without a USB interface that kind of cripples the arp. Keystep has that MIDI In and aftertouch… Novation has pads. It’s a tough call. If Novation would have just added MIDI In and a little deeper configuration, this mini controller would be all over the place in people’s small setups.