How do you plug two computers to the Fireface 400? Only one computer can use it at the same time.

best regards
Knut

If you look at any communications buss specification, no matter whether it is firewire, USB, serial, SCSI, etc., you will find that part of the specification is that there can only be one (1) 'master' controller existing among the connected peripherals. 

In your configuration, there are two (2) masters connected, so the peripheral is attaching itself to the first one detected and ignoring all else.

The other thing that you might want to be cautious of is that firewire (as well as USB) has ability to send power as well as data and buss status comms on the interface.  You are applying two power sources to the firewire buss of the FF400.  I'm not sure whether this could cause electrical harm to the FF400 or to either of the computers, but it certainly has the potential for it.

There are audio communication protocols that will allow you to do what you are wanting to do and that the FF400 supports, like SPDIF or ADAT.  So you could connect one computer to the FF400 through the firewire interface for master control and mixing, then you run the audio signal from the second computer into the FF400 via SPDIF or ADAT and then mix the audio going to the speakers using the RME mixer app on the first computer.

It is worthy to note that even when using audio comms, each of the protocols (SPDIF, ADAT, MADI, etc) require that there is only one 'master clock' on the particular protocol in order to work properly.  In the case of audio protocols, that master can be designated as any of the devices connected, but that there can still be but one master clock in order for everything to sync correctly.

--Dean

I'd like to add an alternate question along these lines. I currently have my DAW, as well as my main day to day computer at my desk, both often up and running at the same time.

Can I assume that I can use the 2nd FW out of the FF400 and connect it to the other computer, providing that I disable the currently active sound driver from the task bar, prior to switching to the other PC? I can't see why this wouldn't work. I just would like to be able to connect either PC while I'm using it so as I can use my studio monitors for either PC.

No you can't.

It wouldn't disable power, for one. You could avoid that by using a 4-pin FW cable, but still the driver wouldn't allow it.

If you connect two macs like that, it wouldn't blow anything up, as they both are on the same FW voltage. If you hook up two random computers like that, it could blow up one or even two FW ports, as the voltage provided could be between 9 and 30 VDC.