The secret with two-legs on the ground at a time gaits is a combo of having wide feet, how big the steps are, and how fast the bot walks.
...Thus with the wide feet, you really have 4 points of contact. You might not actually have the CoG within those points at all time, but it works when moving. My bot can actually stand on two feet because it's centered well enough. Two sucessful 4DoF quads using this approach are Immortal
(mine...). Immortal is small quick steps, while Numa takes bigger steps (I'll probably go smaller quicker steps once I finish IK coding...)