On a motion equation for two particles in relativistic quantum mechanics
  The purpose of the present note is to propose, in the framework of relativistic quantum mechanics, a new Poincare-invariant equation for two particles with masses m_1, m_2 and spin s_1=s_2=1/2. It is a first-order linear differential equation for the eight-component wave function. With the help of this equation the description of the motion of two-particle systems is reduced to the description of one-particle systems in the (1+6)-dimensional Minkowski space which can be in two spin states (s=0 or s=1). 
Wilhelm I. Fushchych (Institute of Mathematics, Kiev, Ukraine)