A. bash
for server in $(cat /home/user/serverslist)
do
for user in $(cat /home/user/userslist)
do
sudo useradd $user
done
done

B. bash
for server in $(cat /home/user/serverslist)
do
ssh -i user@$server "for user in $(cat /home/user/userslist)
do
sudo useradd $user
done; exit"
done

C. bash
for server in $(cat /home/user/serverslist)
do
scp /home/user/userslist user@$server:/tmp
ssh -i user@$server "for user in $(cat /tmp/userslist)
do
sudo useradd $user
done; exit"
done

D. bash ssh user@$(cat /home/user/serverslist) "sudo useradd $(cat /home/user/userslist); exit"