Unfortunately the plans are not capable of running two steps concurrently.
If you are planning to split your group for just one ride, you could just enter one group's activities and have the second group meet up afterwards. For example, enter the Seven Dwarfs Mine Train ride time in the plan and add a note that the others will do Winnie the Pooh at the same time.
Alternatively, you could create a separate plan for the second group, having it include only the hours that it will be away from the first group. This is more complex, but you can make it work.