I do this with my Schitt Mjolnir v3 preamp into a CODA #16 in my office and also into a Schitt Aegir v1 amp (headhones). I have also done the same connections using my Benchmark HPA4 preamp.
I have another Schitt Aegir amp (for headphones) sitting in a box and I would like to hook it up to my Livingroom system that has a Holo Audio Serene preamp and Schitt Wotan amp. There is a bit more danger in doing this one since I was told my the USA distributor not to do this because having both amps on at the same time (accidently) would damage the preamp.
In all 3 cases I am am referring to RCA and XLR outputs connected at the same time. I have accidently left both amps on with the Mjolnir v3 and HPA4 preamps (for and those 2 preamps did not get damaged.