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?