This flares up every so often on CC. I really should let it pass me by but I just seem to be unable to resist giving my three penn'oth
1) Riders in Groups. Absolutely safer to ride two abreast than single file (unless there are big gaps, in which case it's not really a group). Public needs educating of this fact
2) Two riders. At a practical level, not safer to ride two abreast. Fine to do so for social reasons but singling out when there is a car up to maintain good relations with other road users. Reasoning is that singling out doesn't appreciably shorten the overtake distance with just two riders, unlike a big group. Furthermore, the argument that drivers "should" pass a singled out rider as wide as they would pass two riders riding two abreast just doesn't hold water. Reality is that no car drivers do this. Whether they should or shouldn't is moot, it's just never going to happen because it doesn't need to happen. You can pass a single cyclist without going as far over the middle line as you would with two abreast riders