I take it that you only have one set of pre outs, ie some pre amps have both single ended and XLR's outputs. So presuming you only have one I personaly would not do it simply because I would not want to compromise the output going to Amp #1, by dividing the signal with a Y adapter.
As to whether they are dangerous, I would say they are most dangerous to the sound qualiy of your first amp. As to whether it would short out a pre or amp, I doubt it, but I cannot commit to 100 percent assurance.
As to whether they are dangerous, I would say they are most dangerous to the sound qualiy of your first amp. As to whether it would short out a pre or amp, I doubt it, but I cannot commit to 100 percent assurance.