The paper deals with tracking problem for multiple-input multiple-output systems under unknown smooth external disturbances within the framework of the block approach. The paper introduces the notion of joint block-canonical form of controllability and observability of the linear systems with respect to the output. Based on this form, decomposition algorithms of synthesis of an invariant tracking system have been developed. The main feature is using sliding mode state observers for information support of basis laws of combined control. These observers are built as a replica of the joint block form with closed local feed-backs, 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 feed-back in the problem of control of the tangage of an amphibian aircraft under approaching landing under roughness is implemented.