Blue badge / companion badge

Hi,

I'm trying to get my head around the Blue Badge / Companion Badge. Am I right in thinking that you can theoretically use both at the same time for 2 cars (though this might be against the rules, but cannot be easily spotted?)

Background of the question: we've got a disabled neighbour, but it seems that up to 4 cars / vans are parked with badges by people living in this 1 bed flat. Not sure how it works at present, but I believe that one might be the person's blue badge, then a companion badge, then another relative's badge (I know 1-2 are from a different borough) and I'm not sure where the 4th is coming from.