+1 about what DougD said about possible interactions between the two amplifier inputs.Is the only advantage to using a sys in this fashion over just splitting the lines that you can cut the other output? Seems like for most use cases, simple RCA splitters would be the better choice.
There are some other factors, such as the output impedance of the driving source (in most cases, solid-state amplifiers have a reasonably low output impedance, but the same cannot generally be said for all-tube pre-amps / DACs / sources, especially those with a single tube gain stage and no cathode follower (buffer) or output transformer. The length of interconnects and their capacitive loading can also change the frequency response. Also solid-state amplifier input circuits can present a non-linear impedance to the driving source when powered off as well.
TL/DR: There's a lot of factors involved as to the measurable effect of driving two amplifiers with a y splitter: if in doubt, try adding a reversed Sys (or similar device) using short length interconnects.