Hi Peter,

welcome to the RME user forum. You didn't tell what OS you have, I think this is relevant to answer your question. I suggest putting such relevant infos into the forum signature as a one-liner.

Usually no, as USB and FW have two different drivers, and in an application (DAW) you cannot load two different drivers.

Windows: there are "trickeries" to overcome this (ASIO4ALL) which I would not recommend to use because it leads to an unsupported setup. Furthermore, you add an unsupported driver layer on top of the two ASIO drivers. This is against all the reasons why we have optimized ASIO drivers with direct access to the audio hardware. So under Windows, no way.

Apple: might be a different story because they do not use ASIO drivers and support "aggregate devices". But somebody else needs to comment on this. I am not using Apple. I am uncertain whether it is the best practise to work with such aggregate devices or which (possibly unwanted) side effects this might have.

What will work definitively is to connect the Fireface 802 FS to your computer and to connect the older 800 as preamp via ADAT.

If you are talking about "loss of channels". What do you mean exactly?
Because of higher sample rates (double quad speed) and port multiplexing ("bundling")?

The 800 has only 8+2 = 10 analogue inputs (if I remember right). At single speed you can route all analogue inputs across the two ADAT channels which offer to you 2×8 = 16 channels at single speed (44.1/48 kHz).

If you should use double speed, then only 8 channels remain via ADAT. But this might still be sufficient for you.
2 analogue inputs front (mic, instr), 6 line inputs from the rear … sounds fair enough?

But if you are worried about possibly having too few channels for expansion and if you want to avoid having to use two different drivers, then my recommendation would have been to choose an UFX III with MADI I/O.

You could sell the 800 to get some money back to cross-finance the UFX III. Along with that, better preamps and more useful functions like e.g. DURec, Autoset, the display where you can perform all settings on the device even in stand-alone mode.