That was the plan.
When the bus pulled up at the pick-up spot by Rainbow Lodge, the sign in the window clearly said "Phnom Penh." The driver looked at our tickets, waved us on board, and told us that we would be given "transit."
What they didn't tell us was that their method of transit was, once they passed the fork in the road where the main road became two roads going in the opposite direction- one going to Phnom Penh, the other to S-ville - they would stop every oncoming bus and ask them if they had room for Andrew and I and the five other passengers also traveling to S-ville. This turned into a stop about every 5 minutes. And it went on for a whole hour.
Finally after multiple failed attempts to communicate with the driver, we called the SP bus coming from Poipet and discovered they were about 15 minutes away from us, headed our direction. At this point, we were about 3 hours from S-ville, when we should have been only about an hour away from our warm, cozy hotel bed and a hot shower. So the bus driver dropped us off at the side of the road, and the SP bus came rumbling along to save the day. We have rarely been so happy to see our coworkers!
The rest of the trip went rather smoothly. It was hard to complain about our longer-than-expected trip when the SP staff had been traveling from Poipet for about 10 hours at that point. Our trip was still much shorter than theirs!
We are learning to: a) not ever expect to arrive at our destination at the stated time...ever; b) to expect to possibly be taken the wrong destination in the opposite direction; and c) if all else fails, call someone who speaks Khmer.
Those are very valuable lessons to learn when living and traveling in Cambodia.