The paper deals with the tracking problem for multi-input multi-output systems under unknown smooth external disturbances within the framework of the block approach. The paper introduces the notion of the joint block-canonical form of controllability and observability of linear systems with respect to the output. Based on this form, decomposition algorithms of the synthesis of an invariant tracking system have been developed. The main feature is using sliding mode state observers for dataware of basic laws of the combined control. These observers are built as a replica of the joint block form with closed local feedbacks, what enables one to obtain current estimates of components of the control laws directly. Such an approach considerably simplifies the regulator structure, since it does not require direct and inverse real-time substitution of variables, as well as it does not require deriving dynamic models of reference and disturbing actions. Basing on the derived theoretical results, the decomposition synthesis of the feedback in the problem of control of the tangage of an amphibian aircraft under approaching landing under wave disturbances is implemented.