On RnR, each row holds 2 people. Each car has 2 rows. So each car, which is photographed individually, holds 4 people.
On the day we were there, they were stacking loading groups into the individual queues for each row. So for each row, you'd have 6 people, 3 groups of 2, in line for each individual row. As each empty coaster arrived, 2 people would load into each row, and the CMs would then place 2 more people into line for each row. As long as everything goes right, every person in that new group of 28 (I think it's 7 cars per coaster) will wind up loading at the same time. If you're a group of 4, you wind up in two rows next to each other, but load at the same time.
But if someone who is supposed to get into a car holds back, the rows of a loading group are now out of sync. So people who should be loading at the same time, will no longer be.
Think of it this way - the first group on is group A. 28 people, 14 groups of 2. The CMs can then stack up groups B and C, each consisting of 14 rows of 2. As long as everyone loads in the group they're supposed to, they could theoretically go all they way back to group Z, and everyone would stay in sync.
But someone in Group A doesn't get into the car when he's supposed to. Say that's row 7. He's got a Single Rider buddy in group B, back in row 13. So row 7 group A leaves with only one person in it. Now, the holdback guy in row 7 gets into the car when group B loads, and his buddy from row 13 walks over and gets in with him. So the rest of the people in row 7 are now out of sync - the people scheduled to get on with group B are now getting on in group C, the original group C are getting on with group D, while everyone else in the coaster gets on in their scheduled group. That's what happened to us. The CMs didn't plan to split us up, or refuse to let us sit together - instead, since the guy ahead of us didn't get on when he was supposed to, and we didn't notice what had happened in time, 2 of my daughters were in the car while me and my other daughter had to wait for the next one.
I freely admit that had I been watching things more carefully, I could have had my daughters let the people behind them go ahead, or just had them stay back and not get in the car until we were going to load too. Certainly if we ever return to Disney I'll be watching the line like a hawk for any rides with pictures. But that doesn't change the fact that those single riders cheated the system, and I lost out on a good picture because of it.