If both computers use the same ISP address, is there any way to monitor the activity of the other computer, without installing anything on the other computer? (or infecting it with anything)
I would not be breaking the law here.
I believe you mean IP address. In which case, neither computer would be able to access the internet.
Two computers cannot have the same IP address.


I believe you mean IP address. In which case, neither computer would be able to access the internet.
Two computers cannot have the same IP address.
References :