Service Tax can be levied jointly on both the owners only when it is established that they are into this venture as partners. Merely because they are joint owners, or co owners- inference os partnership cannot be drawn. Partnership is a subject matter of individual agreements- it cannot be imposed on individuals.
The joint owners can make a private agreement among themselves specifying that they are not partners and they will be entitled to rental income in proportion of their interest in the property. In that case, both the individuals shall be eligible for Small Scale exemption individually.