It makes no difference whether you issue the ticket as two one-ways or a r/t unless there are advance purchase or non-refundability restrictions on one portion. The airline will refund the unused portion based on the fare calculation printed on the ticket or receipt. The only exception to this is with fares that require a round-trip purchase. Some airlines will recalculate the o/w fare and refund the difference, others will be very lax and refund half the ticket anyway.