Holding is an option even on a road (side-by-side riding isn't) but it
means the other cyclist can't use the brakes well but he or she is going
to have to do most of the braking.

The proper way is described here
http://www.cycleworld.com/2015/09/04...-safely#page-2

The rope goes around the bars and the gooseneck/tripleclamp/whatever and is held to the bars by one hand.

The rope has to be let-go-able.