I've tried googling but can't seem to find any definite info on this even for Linux generally. Does anyone know if it's possible to set up two pairs of bluetooth headphones to receive the same A2DP stream from the N900 so two people could listen to the same audio?

The best answer I've found is this thread that has instructions to set SBCSources to 2 in bluez.conf but I'm not sure it ever worked for the person asking...

So is this possible in linux generally and if yes, then is it possible for the N900? What about getting audio simultaneously to the wired headphones simultaneously with Bluetooth A2DP?

This one doesn't seem to work out of the box, the Bluetooth headphones I have get muted when I plug in the wired ones. Based on a previous thread I tried looking at the mixer controls on alsamixer but didn't find a control to re-enable it unfortunately... Anyone know more about this?

Seems to me like getting the sound to both the wired and BT headset is not something I can do from the mixer but it's something that would be doable with Pulseaudio. I'm just not an expert on it myself and information here on TMO about how it is setup on the N900 is rather slim.

Apparently the module that mutes both phone speaker and the BT headset when wired headphones are plugged is a module for Pulseaudio that Nokia calls the PEP ("Policy Enforcement Point") and Nokia has decided to keep that closed source for some odd reason. Still I'm hopeful the routing could be manually changed with the pulseaudio utils if I just knew how. I think one would need to get the audio routed to sink.hw0andhw1 to get that done. So... nobody a pulseaudio expert here?

bumping this.

having both wired and bluetooth audio at the same time allows me to share audio through headsets with another person.

We need some help here, to understand who is switching from wired to bluetooth audio output, and how we can change this behaviour.

PEP seems to be a good place to start.

Damn research, it's so time consumming

Like you said it certainly should be possible as surely its just the same as when Pulseaudio on the desktop outputs to analog and digital at the same time, as they show up on the PC as two seperate sound cards.

That said, I have had a load of trouble getting that functionality to work on desktop PC never mind when you add bluetooth into the mix which always broke Pulseaudio when I tried to get A2DP working on the PC.

Quite frankly I am amazed Nokia got Pulseaudio to work this well as most Linux distros have seriously broken implementations of Pulseaudio, its quite a well document issue.