Photomultipliers in Parallel
The photons produced by a 2nd scintillator have nothing in common with the photons produced by the 1st scintillator. Neither will a beta particle emitted on one side of a piece of radio active material have anything in common with a beta particle emitted from the other side.

The average rates from the two photomultipliers will be similar at any point in time and therefore, the combined number of "occurances" will be approximately double.