Two sajdahs together comprise one rukn, and if someone does not perform both of them in obligatory prayers in one rakʿah – even if this is due to forgetfulness or not knowing the ruling – his prayer is invalid. The same applies, based on obligatory precaution, if one adds two sajdahs in one rakʿah forgetfully or due to inculpable ignorance (al‑jahl al‑quṣūrī). (Inculpable ignorance is when someone has a valid excuse for not knowing.)