Is it a condition in washing the hands up to the elbows in Wudu' that each hand be washed separately, or is it permissible to wash them together from a single scoop? And is it obligatory to rewash the hands up to the elbows when washing the hands up to the forearms?

Washing the hands up to the elbows at the beginning of wudu' is a Sunnah, not an obligation. So, whoever omits it, their wudu' is valid, even though they have gone against the Sunnah. Scholars have differed on the manner of this recommended washing: some said it is desirable to wash them separately (the right then the left), while others said to wash them together. The matter in this regard is broad. As for washing the hands in the obligatory part [of wudu'], it is an obligation that includes the palms up to the elbows. Whoever washes the area between the palms and the elbows without washing the palms, their wudu' is not valid, because washing the palms at the beginning of wudu' is a Sunnah, while washing them in the obligatory part is a component of the obligation itself.