On theCatalyst 6500 running IOS you can use the "send" command. Don't know if there is a CatOS equivalent.
uc-dist#who
Line User Host(s) Idle Location
* 1 vty 0 craig idle 00:00:00 192.168.57.196
2 vty 1 netop idle 00:00:35 192.168.57.196
send 2
Enter message, end with CTRL/Z; abort with CTRL/C:
Hello Netop
^Z
Send message? [confirm]y
On netops screen on line 2 (vty 1) the message appears:
***
***
*** Message from tty1 to tty2:
***
Hello Netop
