  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).
