The multiple inputs should be isolated from each other with resistances, equal to or greater than the minimum load resistance of the device being considered.
Otherwise, the internal impedance of each source will load down the other source, splitting the levels according to the internal impedances of the sources. Those internal impedances are too low (often near zero), not matched for different sources, and not always constant with frequency.
Y cables or paralleled lines work find to go from one source to multiple loads, they do NOT work well or reliably between multiple sources to one load.
