I use Y cables all the time to drive two amps from one DAC. I then use an A/B switch to connect my headphones to both amps for comparing amps. If there's any attenuation it is not perceptible. There is no loss in SQ as most DACs or line out devices have a reasonably low output impedance and line inputs have a much higher input impedance as to make the load on the DAC negligible, even if two amps are Y connected.
i saw where you posted earlier concerning the a/b switch, good idea, i have only one amp right now, the LD mkiii, but i'm sure that'll change at some point, i like having a solid state amp in addition to a tube amp, i may get another magni 2u, or an a2