It's all about appearance fees for these guys. Both will get a few hundred thousand from that. Yes, the winner gets another few hundred thousand that second place does not, but second place is not walking away empty-handed when it's one of these guys.
As for why they're not running the same race: it may be that they both want to take a stab at the record (as do the race organizers want them to do) and they'll both have an easier time of it if they don't also have to worry about racing each other.
I will admit it's very sad that, in a free market situation, the demand drives people to work toward situations that will produce fast times. In other words this shows that there's a demand for fast times but not for a competitive race. This makes sense when we think about it: much fewer people watch how the race goes than do watch who wins it and in what time. Most of the crowds just notice the final straightaway (by which point, even in a close race, the winner is likely to be all by himself) and the time over the winner's head as he crosses the line. They don't care if second place is a mile behind or if there was an amazing tactical battle that occurred between miles 20 and 25 before this eventual winner pulled away.
Conclusion:
If Geb and Wanjiru race together, we get one of them crossing the finish line in 2:05. That may be after a great miles 20-25 stretch, and that winner might have had to throw in at least one sub-14:00 5k split to pull it off, but in the end it's what's considered these days an average winning time.
If they race separately, we get to see one of them crossing the finish line in 2:03. Then, a month later, we might get to see the other one do the same.
Therefore the race directors, agents, and athletes would have to be on crack to work toward arranging option A as opposed to option B.
if there is an amazing tactical battle between the guys which is won in 2:05:30, then most people won't watch the race