I doubt this is the result of pressure, it would be much more common if it were, at least in a certain region or within a certain ethnicity.

But let’s roll with it and create a fictional pressure. You could have something like a disease that slowly degrades the spleen making very few people reach reproductive maturity. Let’s imagine that having a second spleen means they are much more likely to reach this age, and there’s how you would make the second spleen more common.

Honestly I doubt you could ever have a disease that specific but I can’t come up with a better idea atm.