The last time DH and I used single rider the CM was separating groups and did not send consecutive singles to the same car. Like they sent me to line 1, DH to 6, then the person behind him to 2,etc.
That's what I meant by actively separating groups. If they think you are together then they may send you to different cars even though the same car could be available.
I've never personally witnessed them doing something like that in all of the times I've ridden Test Track. Not saying it couldn't happen, just saying most cast members don't have a vendetta against the single rider line with a goal of splitting up all families.
The only time I've seen something similar would be a sequence like this:
Party of 2 from standby sent to row 1
Single rider sent to row 1 to fill it
(row 2 is still empty)
Party of 5 in standby line sent to rows 3 and 4
Single rider sent to row 4 to fill it
(row 2 is still emtpy)
Party of 4 from standby sent to rows 5 and 6
Single rider sent to row 5 to fill it
Single rider sent to row 6 to fill it
(row 2 is still emtpy)
Party of 2 from standby line sent to row 2
Single rider sent to row 2 to fill it
If the single riders sent to row 6 and row 2 are a couple, they might think the cast member is trying to actively split them. In reality, they were just trying to fill in the seats as quickly and efficiently as possible.
My family of 4 has always had 2 consecutive single riders placed with us in the dozens of times we've gone on Test Track. Many times those 2 single riders were from the same family.