From the For What it’s Worth Department – the General Information suggests you may not be able to get an official pass if it’s just you and your Dad.
In an effort to control traffic congestion on the routes and in the towns, we must limit the number of passes we issue each year. The number of vehicle passes is limited to only 1,500. The vehicle passes are only sold online through June 1 to groups of three or more at the time of registration.
Disclaimer – I’ve never ridden RAGBRAI but just happened to remember reading this. More experienced folks can give you better info.