Levemir has a somewhat shorter duration of action, but with a split dose, it should not be noticible at all. Levemir and Lantus has the same basic strenght. One reference I found suggests that you use the same dosing although some people may need adjustments of dosages depending upon your individual conditions.
No matter what you do, you should monitor your waking number and number before dinner and make appropriate adjustments up or down in your morning and evening injections respectively. You could see how you do for a few days or a week and then make a 5% adjustment (1/2 unit).