A quick search suggests that insofar is the American variant of the British in so far. I always assumed it belonged to the set of expressions like hitherto, heretofore, therefore and albeit. Is there a preferred usage, and does it matter? General context for the usage of these expressions would be helpful.
The form insofar is American English; the standard form in British and other English is 'in so far'.
I also have never seen the form 'in so far' used in American English.