The open relativistic two-body problem, when two interacting particles also are in external potentials, is considered in terms of the principle of the least action. The exact covariant operator equations, which determine the dynamics of either a scalar particle or each components of the 16 component spinor of spin- \(\varvec{1/2}\) fermions, depending on the particle type, in (3+1) MInkowskii space-time are derived in the center-mass and relative motion variables, beyond the consideration in the Breit frame only. The class of external and interaction potentials, when the two-body problem can be covariantly reformulated in (3+1) phase space of relative motion variables, uncoupled from the center-mass motion of such a system, is outlined. In the case of fermions the new \(\gamma\) -matrix basis generating the Dirac-like equation for the 16 component spinors in the (3+1) phase space is found. The chosen basis allows us to decouple the derived Dirac-like equation into four independent equations governing the dynamics of the four-component spinors. The developed approach is examined in studying the low-energy positronium states in magnetic fields.
