Well, this would also not be allowed in my world.... You should have a return trip ! and not 2 one ways.
In this simulation we do not really model passengers making a one way trip and not coming back I believe although in real life that is certainly possible. I see being allowed to make 1 one way trip on every route. In fact I would go so far as to say only 1 one way trip allowed per aircraft. Although you may want to make an exception for longhaul planes
The has to be a simple way of stopping players just massing on HIGH PROFIT routes, (I mean 10 x A330's) from HND to BCN like has been happening in the past, its not fair as the more they mass the higher their ticket prices go up.
Sure Terminals and Hubs bring in additional PAX but thats where this game in the past few rounds has been annoying, seeing massive Jets on one route. There are close to 500 locations to fly to, not just a handful.