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Coupled multi-physics problems are encountered in countless applications and pose significant numerical challenges. Although monolithic approaches offer possibly the best solution strategy, they often require ad-hoc preconditioners and numerical implementations. Sequential (also known as splitted, partitioned or segregated) approaches are iterative methods for solving coupled problems where each equation is solved independently and the coupling is achieved through iterations. These methods offer the possibility to flexibly add or remove equations from a model and to rely on existing black-box solvers for every specific equation. Furthermore, when problems are non-linear, inner iterations need to be performed even in monolithic solvers, therefore making a sequential iterative approach a viable alternative. The cost of running inner iterations to achieve the coupling, however, could easily becomes prohibitive, or, in some cases the iterations might not converge. In this work we present a general formulation of splitting schemes for continuous operators, with arbitrary implicit/explicit splitting, like in standard iterative methods for linear systems. By introducing a generic relaxation operator we find the conditions for the convergence of the iterative schemes. We show how the relaxation operator can be thought as a preconditioner and constructed based on an approximate Schur-complement. We propose a Schur-based Partial Jacobi relaxation operator to stabilise the coupling and show its effectiveness. Although we mainly focus on scalar-scalar linear problems, most results are easily extended to non-linear and higher-dimensional problems. Numerical tests (1D and 2D) for two PDE systems, namely the Dual-Porosity model and a Quad-Laplacian operator, are carried out to confirm the theoretical results.