Here is my double stack 55g. I dato the vertical 2x4 to create a pocket for the horizontal 2x4. I use 3in+ deck screws. You can also use wood glue at all the joints to make it stronger.
I will need to double check when I get home but as I remember dimensions are as follows. 3/4in MDF shelf
L= 49.5in W= 13in
H= 13in bottom of first tank
H= 48in to bottom of 2nd tank
H= 72in overall
The harbinger of doom to the wicked but the fragrance of life for the hopeless. Napalm