Two people can prestige a gym at the same time, which I think raises prestige faster due to this factor: when two people attack a gym together they need to start at the same time, otherwise someone joining later would take a bunch of damage right when joining.
On the other hand, prestiging blisseys by yourself is much harder than two people attacking a blissey together. The same can be said for other tanky pokemon, but it's mostly an issue with blissey. Also, when a blissey or other tanky pokemon is on the bottom of the gym, there's no getting rid of it. You have to prestige it over and over, unlike with attacking where you can do it once and be done. There's no getting around this, but it does make having to do it by yourself even more annoying.
I'm not sure why they made it different for attacking and prestiging whether multiple people can join the same battle.
