Peace be upon you. The Nafila of Dhuhr should be performed before the Dhuhr prayer, and the Nafila of Asr should be performed before the Asr prayer. Source: Sayyid Sistani, 'Minhaj al-Salihin, Book of Prayer, Chapter One on Obligatory and Nafila Prayers.' Sayyid Khamenei, 'Facilitated Jurisprudential Rulings, p. 163.' May you be successful in all good.