Falkirk Council this week became the latest to extend a policy of handing a head responsibility for more than one primary school, saying it could save up to £30,000 each time a shared post is established.
Both the Educational Institute of Scotland (EIS), the country's largest teaching union, and the Association of Headteachers and Deputes in Scotland (AHDS) said that the number of shared headships had risen significantly in recent years.
The Accounts Commission said this week that local authorities, which have been forced to make substantial budget savings in recent years, had cut spending on education by five per cent in real terms between 2011 and 2013.
The EIS raised concern over the policy, saying that it believed every school should have its own headteacher.
A spokesman added: "While it is perhaps understandable that, at a time of falling budgets, a growing number of local authorities have explored shared headships as a cost-saving measure, the EIS does not believe that financial concerns should necessarily take precedence over educational concerns."
Greg Dempster, AHDS general secretary, said that there were more than 200 schools in Scotland with shared headteachers and that the practice had become much more widespread in recent years.
In a report, Nigel Fletcher, Falkirk Council's acting director of education, said: "These kind of joint management arrangements are becoming increasingly common across Scotland.
"Given the current financial situation, it is imperative that as a service we explore every opportunity to make efficiencies."
The council is not required to consult parents, staff or unions on management plans for schools, although the authority has said views of parent councils and workers' representatives will still be sought.