The main reason that Chessington don’t pair up groups of two on Tiger Rock is mainly because of faff.
Being a family park, most family groups would rather ride without strangers and will likely argue with staff at the batching point if they try to pair them with other twos. The conveyer in the station can only be stopped for maximum of 15 seconds. If it stays stopped for longer, this automatically shuts the ride down. If guests were told to pair up with strangers, they would most likely stand there arguing, the staff wouldn’t want to stop the conveyer, so by the time the staff had finished arguing with the guests, 2-3 boats would have gone round empty, meaning it would actually be worse for throughputs.
There’s signs in the station warning guests that they might be paired up, but most people don’t read those signs and families tend to be the most demanding and entitled demographic, who would be the most unhappy about having to share a boat. Certainly when I worked there, we figured it was just easier, and more efficient, to allow small groups to have boats to themselves.