FCPX doesn't have a "send to LogicProX" as far as I know.
There's not that level of integration - other than with Compressor.
But you can easily export audio and then import it into LogicProX, manually.
Then export it from LogicProX back into FCPX.
I think a lot of people are hoping for the sort of integration you speak of - especially with Motion too. Might be a way off.
The XML export to Logic seems to be working much better than it was.