Hmm... that shouldn't have been the case. Not in Windows at present, so I can't test it here (well, not *strictly* true as I could RDC to one of the servers, but I don't have a headset here either...) Give me a half hour to get into Windows and check...
Later: well, just back from Windows and it all worked exactly as I had thought it would. Connected to IVAN server (France, it was autoselected) as EGLL_OBS, used "Request ATIS" to get EGLL_A_TWR's ATIS. Scrolled up through that to get his TS2 server/channel info (gb1.ts.ivao.aero/EGLL_A_TWR)
Manually connected to gb1.ts.ivao.aero in TS2 client (using my IVAO VID and IVAN password, remember they have different passwords for the network and for the website), set mic mute in the UNICOM channel, then moved myself into the TWR channel, and he was just handing off a pilot to uncontrolled airspace as I joined, so I know that the TS2 "obs" was working.