The new Dynamips version allows you to open multiple consoles mapped to the same 'line con 0' port (and also supports the same functionality for the AUX port)
So, if you open a new console to a router, Dynamips will share the 'con 0' line with all others open connections to that port, and if you type a command to a active connection pointing to a Console or AUX port,
the output will be displayed to all connections sharing that port (Console or AUX), but not on both Console and AUX (same on real device)
Now, with Qemu is another story I've described something here
<- (but is related to Qemu Host, Linux OS, and not ASA).
Anyway, ASA is also running Linux (BusyBox) and probably can be configured as well to support multiple consoles (I've never tested)
But don't forget that opening multiple consoles for Qemu is not fully supported by GNS3 (yet)
Now you can open as many as you want, but Qemu permits a single TCP connection to a console port, the rest will simply not work... (And if you open/close the console a few times, will also kill it ;)
To fully support multiple consoles for Qemu, GNS3/Qemuwrapper should start Qemu like this ex: Con1 "-serial telnet:127.0.0.1:3001", Con2 "-serial telnet:127.0.0.1:3002", etc... and then open each console to a unique TCP port and never share the same port!
Also the device (Linux) needs to be configured with multiple consoles ports (ttyS0,ttyS1,ttyS2,etc...)
PS: Qemu also supports VNC, but not for ASA ;)