When there are two verbs in a sentence, can one auxiliary function for both? If the two verbs take different auxiliaries, can one still be employed for both? I ask because sedersi is reflexive and takes essere, where fissare is not and takes avere. Fissarsi doesn’t seem to apply since it doesn’t mean to stare, but to obsess. I want this sentence to be “Mi sono seduto e l’ho fissato.” Is one of my presumptions wrong here or is this sentence ungrammatical?
It does indeed look ungrammatical. I wonder where you found it, because I can’t find it in any collection. I can find the English sentence on Tatoeba, but according to the logs it has never had an Italian translation. A mystery!