In a report on small schools, Estyn had questioned whether having a single head in charge of a cluster of schools "can be accommodated within the interpretation of the current statutory requirement for each school to have a headteacher".
It wants the Assembly government to consult on a new notion of "shared headship", which would be the managerial equivalent of having a single governing body for a group of federated schools.
The government agreed this week that each school must have one named head - but said a head could "legally act as the head for more than one school".
A spokesperson said: "This is more likely to happen in small schools, and in temporary arrangements, where the head of one school also becomes acting head of another if a vacancy arises and cannot be filled. But the latter situation should not run indefinitely. It would also be legal for two separate schools to share a head if both posts are part-time."
Estyn's report says formal clustering of small schools (defined as 90 pupils or less) under a single head can result in significant improvements to management, teaching and learning.
Existing and sometimes long-standing "shared headships" in Powys, Carmarthenshire and Anglesey have arisen in response to local circumstances - usually when one school struggling to find a new head is taken under the wing of a neighbour.
But clustering may become more prevalent, as some local education authorities try to retain small schools while dealing with falling pupil numbers.
Magnus Gorham, assistant secretary for salaries and pensions at the National Association of Head Teachers, said none of its members had raised concerns about the legalities of taking on responsibility for other schools.
But he said: "Our concern would be the head effectively taking on two headships, two schools, two governing bodies - two everythings - and how that is translated into pay and conditions," he said.
"Local authorities are offering members one or two extra pay points, and we don't feel that reflects the extra responsibility."
Meanwhile, the Assembly government has yet to enact regulations allowing schools to work together under a single governing body. A consultation is expected this summer, with the new rules coming into effect in 2007 - three years after England.