Why are there two such organizations? I mean, I know the federal government never stops at one bureaucracy when they can create multiple, but what are the effective differences in focus?

It's more accurate (IMO) to say the two have really different operating models. 18F is funded via cost recovery - agencies have to pay per project via an official agreement between agencies for the work (which can make it harder to do a project). USDS is funded via congressional appropriation, so doesn't need the same types of agreements.