Yet they share a site on the edge of Cumbernauld, in North Lanarkshire, and it is by establishing common ground that they received recognition at last month's education awards. The schools won the "better behaviour and attendance" award, having each adopted one of the other's successful schemes.
Previously, there was the potential for confusion. They share play areas, a dining hall and a games hall, as well as a janitor and other staff; many pupils travel to school together. If the site was home to a single primary school, it would be one of Scotland's biggest Cumbernauld has 610, while St Andrew's has 230.
Things could get messy when Cumbernauld ran a house points system on its own, where the school's pupils were divided into four competing groups. Classroom assistants, asked to award points for good behaviour in the playground, could find the exemplary conduct they had picked out was that of a St Andrew's pupil who had no idea what "points" meant.
In October last year, that situation was redressed when St Andrew's adopted the same house system. Now each school regularly updates house scores with the best-performing house each week receiving 15 minutes extra playtime and, at the end of the year, there is a grand total combining the scores at both schools.
"It means it doesn't matter what school you're in," said Marie-Claire Fiddler, acting depute head at St Andrew's. "The house really straddles the two schools."
Holly MacLean, 11, who was a house captain last year at St Andrew's Primary, says the system has influenced behaviour: "You wouldn't want to be the person that lost house points."
At the same time, Cumbernauld took on a St Andrew's scheme the HABIT award homework, appearance, behaviour, interest and time-keepingattendance. This is more classroom-based, and the best-performing class each week receives a Pounds 10 voucher to spend as they choose, while the class of the year wins Pounds 200.
Behaviour has since improved, and most pupils wear school uniform without fail. "If you look around and everyone has school uniform on, then I know we won't lose any points," said Holly.
Another benefit of HABIT is that supply teachers spend less time taking control of a class. "Children play up a bit when supply teachers are in, but they have said there's been a big improvement," said Simone McCredie, principal teacher at Cumbernauld.
Holly said she would often walk to school with pupils from Cumbernauld, who would be eager to swap notes on progress in both the house system and HABIT. "In the games hall there are two big boards (with house scores) you see Primary 3s running and checking boards to see if they're winning."
The joint practices are matched by a close bond between teachers in both schools. Although there is a partition that could divide the joint staffroom, it is never used. The schools' management teams meet weekly, and the two heads have a close working relationship. Staff work together through joint after schools clubs and often socialise together.
St Andrew's was created in January 2003 after the shared site opened. Staff believe this partly explains the successful relationship between the two schools, comparing the difficulties on some other shared sites where both schools had previously existed elsewhere.