What I'd do to accomplish what you're planning to do is this:
Get three 48 point patchbays.
Patchbay 1:
Wire the analog Tape Outs 1-24 from the D8B to the top row on the back of your patchbay.
Wire the analog Inputs 1-24 of your Tape machine to the bottom row of the back of your patchbay.
Make sure each patchbay module is set to "half-normal" (or "normal" if you prefer - if you don't know what that is, I'd suggest to search for "half-normal" online). The result will be, that the output of your D8B, will be routed to your Tape Machine by default, and when no cable is plugged in.
You can always plug in a cable, and send the signal somewhere else, instead (e.g. your audio interface... more on that in a moment).
Patchbay 2:
Same as patchbay 1, but with the Tape Machine Outs 1-24 on the top row (in the back of the patchbay), and the D8B tape inputs on the bottom row (again, in the back).
Make sure the modules are set to normal/half-normal.
Default routing will be that your tape machine sends the signal to your D8B's tape return inputs.
Patchbay 3:
Wire the analog outputs of your audio interface to the back of the top row.
Wire the analog inputs of your audio interface to the back of the bottom row.
Make sure that this patchbay is NOT normalled or half-normalled, or you'll create a feedback loop.
Get 24 (or 48) patch cables. If you want to send signals between the D8B and audio interface, or the tape machine and the audio interface, it's just a couple of patch-cable connections away (e.g. patch D8B Tape out to Audio Interface analog in, etc.).
It might be a bit pricey to get this many patchbays and the necessary wiring... but then you'll be operating in "Pro" territory.
If you add more external hardware and put it on another patch bay (or open patch points on the ones you already have... e.g if your interface doesn't fill up the 3rd patchbay), things get even more flexible, and you can patch whatever hardware you might want to use into your signal chain via the patchbay.
If you ever got used to using a patchbay, you'll never want to go back. In fact, the main reason why I still use a D8B, is because it has separate XLR and TRS Inputs/Outputs.
With the XLR/TRS combo jacks as found on many newer digital mixers, you can't really put them on a patchbay, if you want to use both, the XLR and TRS connectors (you always have to go to the back of the mixer to change connections - which in my case would mean to open a giant lid on that Argosy desk I have... not practical at all).
That's why I still use the D8B: Being able to use it with (a) patchbay(s)