My colleague Kiran Patel, has showed me this really cool command:
superdiffcommand='systemctl list-units --type=service | grep running' && sdiff -w 200 <(ssh box1.com $superdiffcommand) <(ssh box2.com $superdiffcommand)
If the command you want to run requires root priveleges, then you can do:
superdiffcommand='sudo systemctl list-units --type=service | grep running' && sdiff -w 200 <(ssh -t box1.com $superdiffcommand) <(ssh -t box2.com $superdiffcommand)