I think one should practice scales hands separate and hands together, for different purposes. Hands together scales help develop coordination and perfect timing of the two hands. It can be harder than you might think to get the corresponding notes in both hands to sound at exactly the same instant. Hands together you can play around with contrary motion, scales at the sixth or tenth, scales in different times (ie 2 octaves in eighth notes in one hand together with four octaves in sixteenths in the other).
However, if you want to increase speed, it's better to work a lot on hands separate. Getting the motions of each separate hand just right requires concentration on one hand at a time. Working on relaxation, feeling the weight, and all that are also best, I think, if you start the scales hands separate. Then you can put the hands together if you like.
Here's a link to a good, older post from Bernhard about scales:
https://www.pianostreet.com/smf/index.php?topic=9211.0Good luck.