An ERJ crew took the runway for takeoff with only one engine running and so had to exit the runway to start the second engine for departure.

Narrative

We were going departing on a day when the wind was light from the south. Upon briefing the departure at the gate and discussing our probable long taxi on the push back we decided to start only the number one engine. The usual taxi is at least 12 minutes long from our gate. We did the single engine after start check; called for taxi from ramp; taxied to the top of the ramp; called metering; were contacted by ground and subsequently assigned XX for a Departure Runway (about a 4 minute taxi). We taxied without stopping or any other event to the XX. We completed the before takeoff check and were assigned position and hold from tower. We were the cleared for takeoff about while we were crossing the hold short line; which I accepted. As we were making our turn to line up with the runway; the Captain noticed that we had never started the number two engine. We requested to depart the runway and we started the engine on the way back to XX without any further incident. We did the checklists; departed with a long left turn to a heading of about east. As for suggestions; our destination is a very fickle and ever changing environment. After up to seven legs a day on each trip we get complacent because we think we know what to expect; but just like at any airport we can be assigned what we do not expect or changed half way through the taxi or at any other time. We normally start two engines when we expect the short taxi. Maybe we could do the departure briefing after we receive our assignments or just prior to takeoff instead of at the gate guessing as to where we think we will go. Also; we need to realize when we are at the end of a long trip that we have a tendency toward complacency especially on good weather days when everything seems to be going right.