There should be no trouble networking Moodle 2.0 and Moodle 1.9 together. However ***please test it thoroughly, well in advance** (for maximum points of helping the development team, do it while Moodle 2.0 is still in beta
Having said that, the new MNET test client you find in the administration -> development tools item in the admin block in Moodle 2.0 will not work properly with Moodle 1.9. However, this is just for developers, so shouldn't affect you anyway.