I tried both ways with Lantus which is supposed to have an action time of 24 hours, but it definitely worked better for me to split the dose and take it twice a day.
I now take Levemir twice a day; I haven't tried one dose a day with Levemir since it is definitely not supposed to work well (the action time of Levemir is only about 12 hours).
I would talk it over with your doctor. You may find that 12 hours apart may not work the best - I take my evening dose at 10pm though I take my morning dose at about 8am. For many people like me, taking the evening dose later can prevent an apparent "dawn phenomenon" from occurring.