Well.. the short answer is no. However, you can still "hack" it, but it's totally unsupported (and will most likely break during upgrades). The reason you're not able to run two instances is that it uses a Jetty server and a prism browser to connect to it on the local machine. By default it uses port 7633 so any additional zdesktop instances will conflict ports. I did make it work, but it was more of a "because I can" than actually wanting to use it that way. The setup looks something like this:
1. install zdesktop for the first user
2. stop the zdesktop services: ~/zimbra/zdesktop/zdesktop stop
3. change the port in the client and server config files - you can find the times the port is mentioned in config files via the command:
grep -r 7633 ~/*
I changed each instance of 7633 to 7634
4. start zdesktop: ~/zimbra/zdesktop/zdesktop start
5. launch the zdesktop client and verify that you can still access it
6. logout as the first user and login as the second
7. install zdesktop like normal
That seemed to do it for me.. But, like I said.. more of a "because I can" reason than actually being a permanent solution
