Good luck with that, since Avid make the worst core audio drivers out today, well maybe not as bad as M-Audio... oh wait, that's Avid as well, doh.
Q are you talking about the rackmount version or the "mixer" version of the 02?
The "link" is simply a few cables between one device and the other. The challenge is figuring out how you want to do it so it works the way you need it to work.
So Q: how do you see the 2 devices being used?
The point I'm making here is: with such an entirely vague question for such a potentially complicated answer. We simply require more info on what exactly you have, and how you wish it to work. and please use specifics like "I want to take the direct outs from the Digi 023 and hook them up to the inputs on the mackie, then take the buss outs of the mackie and connect them to the Digi 02. How do I make sure I don't get a feedback loop happening if I decide to work like this?".
Do you want to use the Mackie for monitoring, or to simply use it as channel strip like inputs so you can have EQ and whatever BEFORE you go into your system, or maybe you want to use the mackie as a way to combine a bunch of tracks together while you record (say a bunch of keyboard sounds from some rackmount synth).
give us more info, I'm sure we can help...