During my first ten years studying classical piano, the scales that I played were hands together, parallel. But the only literature that actually has scales in both hands, for more than one bar per page, is quite advanced and non-solo: Brahms piano trios, Beethoven piano concertos, and maybe some simpler Mendelssohn piano duets. But nothing that a kid would play in a recital.
If the point of drilling things like scales is to prepare the student for the solo keyboard literature from Bach through Liszt, then why not one hand at a time, as scales actually occur? Learning one hand at a time has many advantages, which I won't belabor.
On the other, ahem, hand, if the more important goal is hand independence, then why not drill right hand C major in triplets against left hand F sharp minor in sixteenths starting five notes late? Or at least against an Alberti bass, as in the Mozart sonatas?