Both sides can accommodate instrument inputs (and if you use an instrument with phantom power via a TRS connector, it will do that at various voltages, as well as the mic input which will do a full 48v).
I used my Felix (Serial #4!) for mixing a series bass a lot and it works very well (although the gain isn't quite the same on both channels as they are optimized for different types of inputs, but it doesn't really matter because the bridge pickup has less output anyway). It's super flexible and the sound quality is fantastic.
I've also used it from the individual outputs of an F2B. At some point, I want to look into the 2nd version, as it has stereo FX loops, probably because I have been bugging them about that since the first one came out.
It's a veritable audio swiss army knife! Let me know if you have any more questions about it!