Same as what I was thinking. A short ride with a mate to meet for a coffee is fine, but on several day rides two bikes put too much of a strain on things. You're never ready to go in the morning at the same time, almost never really want to ride at the same pace, get tired at different rates, etc, the freedom of going out by yourself in the morning is invaluable to me on trip. Having said that, there are people that just click together perfectly, be it couples or friends who are very much in tune and even when they're not in tune it's still cool, but that's rare to find.
Going for a ride with absolute strangers is a nightmare to me, having said that, I might be planning to lead a mototour through northern Portugal next year and see how that business goes