Technically there are ways of doing a mass join using multiple hosts, but it's not the simplest method, and correctly identifying a mass join that uses multiple hosts vs. a lot of people joining at once (such as after a net split) isn't simple either.
Generally speaking, it's unlikely that you'll actually encounter a situation where one person tries to do a mass join using multiple hosts.
I've been an IRCop on a couple of networks for a couple of years now, and, to the best of my recollection, I've only seen it happen once.
