Backing up networked iMacs
You can use one USB drive to back up multiple OS X Macs. Just connect the drive sequentially to each. 

You could cycle the drive daily, weekly, monthly or hourly.

Each backup is kept separate and yes, anyone can read the backup files from either computer if they are an administrator of either Mac.

You could also make use of Crash Plan software to back up each Mac to the other over the network. That way you could have Time Machine on one Mac cover both of the computers - one directly and the other backing up the crash plan data.