It is possible, but I'm pretty sure it won't happen ...
How I would expect The Conduit to outsell Killzone 2 would be for it to sell moderately well initially (1 Million in the first 8 weeks) and then to see steady sales over time. Why the Conduit might have longer legs is (primarily) due to lower competition ... Killzone 2 will have to compete with Resident Evil 5, Chronicles of Riddick, ShellShock 2, and Fear 2 which were all released in a similar timeframe and appeal to people who are looking to shoot things.