Air carrier First Officer reported the aircraft veering when power was applied for takeoff when it was discovered the right engine had not been started. The crew taxied off the runway; started the engine; and then departed safely.

Narrative

As CA started to increase both throttles for takeoff; the plane experienced an immediate pull to the right. CA immediately reduced the throttles idle and both CA and I did a quick scan of the engine instruments and discovered that the number two engine was off. CA had me request cancellation of takeoff clearance and taxi off the runway at the next taxiway. I advised ATC that we did not need any assistance and needed a few minutes before taking off again. Once clear; I started the number two engine and we re-accomplished all checklists from the After Start Checklist to Before Takeoff Checklist.Cause: Lack of communication; improper checklist usage; distraction; and complacency. CA did not brief single engine taxi and I failed to double check with him if he meant to only start #1 engine on pushback. This led to a breakdown of our shared mental model. Distraction of taxi and communications; mixed with complacency; led me to have a memory slip and think that I had already started both engines. When CA called for two engines; before takeoff checklist" I did not visually cross check that we had two engines; instead I was heads down in the checklist/takeoff numbers/FMS.Suggestions: 1) More assertiveness and better communication when unsure of CA intentions 2) Physical reminder on scratchpad or notepad when taxiing single engine 3) Visually scan engine instruments along with the CA when performing Before Takeoff checklist."