Having had an OT for years, and just recently switched to the Digitakt, I'd say that owning both doesn't make a lot of sense. Yes, they're very different, and the one doesn't really replace the other. It's just a matter of how you prefer to approach samplers.
But having both, they still overlap to a great extent that I think a lot of the point with the Digitakt would get lost. For me, its convenience and immediacy as well as slightly more focused approach, triggers other things within me and thus, makes it more appropriate.
But if I'd have both, that convenience would be lost since I'd still be in Octatrack land. And I really liked that land. So if I'd decided to be there, or go back there, there's nothing the DT would do that I'd felt was strong enough to keep both.
The main differentiator is the DT's convenience and all that comes with it. Not the features. Pair it with the OT, and you'd get a massive overlap of features and lose the convenience. Sure, it has a different sound. But it's not that different. And it's not like the OT sounds bad. It sounds bloody great.
Obviously, to each his own. But if you have an OT and like the OT and prefer the OT over the Digitakt, but could see points in having both anyway, then maybe I'd say, think twice about that.
Not that you actually asked but here I go again, anyway.