careful when using fast lenses... big apertures create shallow depth of fields.
so sometimes you might get pictures where the focus isn't where you want it.
or if it's a group shot some people would be in focus while those right beside
them are already getting soft. sometimes it's just better to work with higher iso's.
having two bodies does help eliminate missed shots from lens switching but
it's really bad for your back

a 17-70 lens or anything in that range is enough
to suffice a whole event. if you find you can get good shots out of an 18-200
then go for it.