You will probably find Layers the easiest way. In the lower lefthand corner of your Finale screen you should find four numbered buttons corresponding to the four layers available. Layer 1 is the default; you will want to use Layer 2 for the lower notes (and rhythms).
You may find the same idea useful on the lower staff as well: Layer 2 for the tied note and Layer 1 for the note on top. Since one can use all four layers on each and every staff, one seldom needs to use all four layers.