2 is fine for the 20 longs. Cinder blocks are how i do my setup, the only catch is cinder blocks are only 16in but a 40 breeder or a 75 is 18in, so I use a standard cinder block a small gap of about an inch, and a half cinder block. This way i get 2ft deep shelves so I can have 10s and 20 talls narrow ways for space savings, as well as 40s and 75s long ways.
Also don't forget tanks are usually 1/4in to 1/2in longer than is normally thrown around, give yourself some breathing room on the shelves, it stinks to have to tear it all down because you didn't account for this, may possibly be speaking from experience...