Yes it is possible, with a tool named ttyecho that can emulate user interaction in different terminals.
Download and install:
wget http://www.resulinux.tk/utils/ttyecho.c -O ttyecho.c
gcc ttyecho.c -o /usr/bin/ttyecho
Now lets execute something in other terminal, for example as user john that is loggeg at pts/17 using xterm as you see in the ps command:
ps aux | grep john
john 9198 0.0 0.0 23836 4524 pts/17 Ss Jul21 0:00 /bin/bash
Now, lets try to open vi and type some text in the other terminal.
ttyecho -n /dev/pts/17 vi (executed vi command on the other terminal)
ttyecho /dev/pts/17 i (entered in insertion mode)
ttyecho -n /dev/pts/17 some text
When you look to the terminal that john is logged in, you will see that vi is really executed, and you can see the text we type at it "some text". So now you have full control to the others terminal sessions.
Running commands in multiple ssh sessions
Determine the ssh pts devices:
ps aux | grep ssh
root 3540 0.0 0.0 44924 5764 pts/1 S+ 14:46 0:00 ssh firstname.lastname@example.org
root 5907 0.0 0.0 44924 5684 pts/17 S+ 12:51 0:00 ssh email@example.com
root 8074 0.0 0.0 51216 3948 pts/6 S+ Jul26 0:01 ssh firstname.lastname@example.org
So to execute commands on ssh logged in 192.168.2.77, i just need:
ttyecho -n /dev/pts/6 ls
And the ls command will execute really remotely in the single script!