For a co-axial rotor, a bigger/taller rotor mast is needed. There is extra parasite drag from this. In higher speed flight, a tail rotor can be offloaded by good aerodynamic design of the tail empennage.
A co-ax is good for low speed lifting because there is no "wasted" tail rotor thrust.
Two separate main rotors are even better because there is less airflow interference between them.
As always, design is a compromise.