Hi,
If you're Trimix certified (and provided your SAC is not excessive), a 2x12L twinset is sufficient for dives down to 70-80m. Consider larger twin-sets only if you want to dive deeper or if your SAC is higher than average or if you plan extremely long dives.
Kind regards.
Cyrille
PS: for the double 20L you need a crane ... to gear up and to get out of the water
