When supplicating for one's parents (for me and for the Muslims) in prayer, is it better to use the plural form "al-walidayn" (the parents) or the dual form "al-walidayni" (the two parents)?

It is preferable for the Imam to supplicate for himself and for the congregants and seek forgiveness for everyone, based on the Almighty's saying: "So know that there is no deity except Allah and ask forgiveness for your sin and for the believing men and believing women," and based on the Prophet's (peace be upon him) saying: "Whoever seeks forgiveness for believing men and believing women, Allah will grant him a good deed for every believing man and believing woman," and also based on his saying to 'A'ishah (may Allah be pleased with her): "By Allah, this is my supplication for my Ummah in every prayer."