In-movie logic: Darth Vader will be looking for twins, not single children, and splitting the pair makes it more likely that one would remain free in case Darth found the other.What is less logical is that Obi-Wan selected as foster parents couples known to Anakin/Darth rather than some complete strangers. Maybe he decided that Darth would think it unlikely that he would do something that stupid.
Out-of-movie logic: When the first movie was made, George Lucas had not yet decided that Luke or Leia were siblings, nor that Darth Vader was their father. The latter was decided during the making of The Empire Strikes Back (it is not in the original script by Leigh Brackett, where a force ghost of Luke's father shows up). That the two are siblings wasn't decided until the making of Return of the Jedi - when Obi-Wan says to Yoda, "Remember, there is another" in TESB, Lucas had not yet decided who that "other" was.