Then your companion pulls out that typical black umbrella, probably bought from one of those strategically positioned men around Oxford street on another rainy day when they were as forgetful and stupid as you. And then it happens! The attempt to share the umbrella.
You first try to figure out who's best to hold it (probably the tallest one). Then you try to get as close to each other as possible without depriving each other precious oxygen. Then you try to position yourself in a way that you won't have the accidental bum-to-bum slap when you walk. But it doesn't work; mathematically it DOES NOT work. And it will NEVER work [unless you carry a tent].
Umbrella sharing is a useless negotiation process and, quite frankly, annoying. You always end up with two half-wet people. It's better to have one perfectly dry, comfortable, and probably evolutionary fittest person. I rest my case.
Painting by Gordon Bruce at http://gordonbruce.deviantart.com/