This is addressed in the film's official novelisation. Obi-Wan suggests that they be split up as allowing the discovery of both (at the same time) would be an intolerable error on their part.
“We should split them up,” Obi-Wan said. “Even if the Sith find one,the other may survive. I can take the boy, Master Yoda, and you takethe girl. We can hide them away, keep them safe-train them as Anakinshould have been trained-“
In the film the dialogue is reversed (Yoda suggests they be split up without explanation) but I think we can be reasonably confident that the reasoning is much the same.