ORIGINAL: ptebbe
Is there a way that someone could be "at fault" for causing a mid-air collision?
Yes, but it tends to not be very common. To be at fault, you would have to be violating some rule... e.g. let's say the club has rule that states that landing aircraft have priority over takeoffs, and then person X clearly calls "Landing", and yet person Y still heads out and does a downwind takeoff right into X's aircraft as it was just about to touch down... well then Y is at fault.
The majority of mid-airs tend to be no-fault issues though - no-one busting a rule, just bad luck. Using a "spotter" can held reduce the likliood to some extent, as long as you can remind your spotter that he is supposed to watch for other traffic, rather than just stare at your plane
Ironically there were only two aircraft in the air at the time.
That seems to be common. We had a discussion about that at the club a while back... only thing we could think of to explain it, is that when there's more planes in the air at the same time, people tend to pay more attention and stick to predictable flight patterns (and maybe take a spotter along), than when there are just 2 of you out there.
Gordon