Say I have a particle moving along the $x$-axis in the Earth's reference frame. It decays into an upsilon and a proton, each of which has an energy of 60 GeV. They are traveling in opposite directions. The proton has a mass of 1 (or 1GeV/c^2) and the upsilon has a mass of 10 (or 10GeV/c^2).
My question is; can I set the four-vector of the original particle as:
$(E, Px, Py, Pz)$
And the four-vector of the decay particles as one general vector:
$(E', Px', Py', Pz')$
Such that $E'=120$GeV, the total energy of the two decay particles? Or, to find the energy and momentum of each particle, would I have to have two separate four-vectors and calculate them using the inner product?