Jarmo describes setting up the DM4800 as a "Split Console." I do it a little different. I need to mix a show and track at the same time, this method lets me tweak my live mix by recording the rehearsal. You have two routable layers of selectable inputs on each channel. You can see the two boxes for each channel on the input page, the lower one is called "Returns." For the first 24 channels, I would route the analog ins 1-24 on the top input, and your TDIF in 1-24 to the returns layer. I would also route CH 1-24 out to TDIF 1-24 using the *input bypass* routing function, to send the micpres out direct to the recorder before the faders, eq, comp etc.
Ch 1-24 also go to the stereo outs. This way you can eq, comp, tweak your inputs as a working mix, and not affect the sends to the X48.
Now on playback, you can flip the inputs 1-24 in groups of 8 at a time to the TDIF ins on the return side input. Now when you playback, all the tracks sound the same as they have the same eq, comp, etc. applied. I use this method to do live mixes while also recording. When I record the rehearsal, I can tweak it for the webcast or a house mix in the space before the show. Or even take the recording with you to work on a mix in a new venue without the band there.
It would be nice to be able to flip between inputs to returns in groups of 24, but I don't expect Tascam to update the software and how many people track 24 at a time anymore? This method does not use the 24 busses or 8 groups, so you can use the groups for mixing and are not limited in how many channels you can track at once. Except by the number of micpres you have of course! You will need some 8 channel micpre units with digital outs to get into the DM4800 to use the full 48 tracks on your X48 for tracking and a couple cards maybe.
HTH.