I saw it mentioned on plenty of forums but no-one said what it's all about.
So I searched on amazon for a book about autocare which you could peek inside. Sure enough just as "that wigginer" said, you have one bucket with the suds and the other to rinse off the muck you've just washed off the car.
Kind of makes sense, and it only cost me £2 for another bucket.
I'll have to see how mine handles the bugs this weekend, (car covered in them) but I have got some "bug and tar" remover I'm testing too.