大乘百法明門論開宗義決 1.68

1.68

然亦不許一念並生者。小乘論云。以一身中無二心故。等無間緣唯有一故。是故不得一念並生。

Regarding 'However, neither is it accepted that they arise simultaneously in a single thought-moment': the Hīnayāna treatises state: 'Because there cannot be two minds within a single body, and because the immediately preceding condition is only one, they cannot arise simultaneously in a single thought-moment.'