Hi,
You Tagged and Categorised the Post with Messages
Categories: Messages, Using OS X Mountain Lion
Tags: screen, lion, external, messages, mountain
I presume you are talking about the Messages app.
To start it off you need to list an Apple ID that you want to use with the iMessages Service.
This can then be used to send and receive iMessages from iOS devices or other Messages apps
The app can also use the older style iChat Screen Names and IDs and these can be added the same way they were in iChat.
The "iChat Side" is the bit that does the Screen Sharing and it should cope with some one using two Displays.
What you have to remember is that two side by side displays may have to be shown on one 15" monitor and everything can look very small.
10:20 PM Thursday; September 6, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously