I don't think you're supposed to just average the two together somehow by themselves. The two signals are designed to be used in different zones and in practice have very different signal strengths at any point on approach.
From EVALUATION OF INSTRUMENT LANDING SYSTEM DDM CALIBRATION ACCURACIES [sic] by Dennis M. McCollum [PDF]:
The course pattern ...
When an airport has multiple ILS approaches to the same runway (usually labelled ILS-Z, ILS-Y, ILS-X, ...), they all end up using the same ILS for that runway, but they differ in how you fly the approach until final.
Consider e.g. KTUP, which has an ILS-Y and ILS-Z approach to runway 36. Here is the chart for the ILS-Y:
This ILS approach starts ...
ATC will assign the correct approach.
There are many different reasons some airports have multiple different ILS procedures for the same runway:
Some have separate procedured depending on whether ATC will provide vectors to final approach or the pilot needs to fly a procedure turn
Some have different procedures because of terrain/obstacle clearance - ...