y cables, are they dangerous

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.

transporter, i still have a xlr output available but didnt know if the signal would overlap or cancel out the rca connection in the other amp.

my preamp has 3 rca amplifier outputs which are all being used.

im currently biamping a pair of klipschorns with a mcintosh mc 7104 ss amp on bottom & mc2102 tube amp on top & this takes 2 of my outputs.

the 3rd output is running a mcintosh mc300 ss amp powering mcintosh xrt22 speakers & i want to biamp them also with another mc2102 tube amp for the tweeter colums.

my preamp has 1 xlr output that isnt being used,do you think this is the route to take as opposed to the rca's.

im smitten with the results of biamping the khorns & very much want to use another mc2102 on the tweeter colums of the big mac's as the benifits were so obvious.

mike

Honestly Im not sure in this particular application. One issue is will the XLR and RCA outs work simultaneously, my guess would be yes. But the stumbling block is that XLR outputs almost always are at a approx 4db higher output, and this issue would have to be addressed. Lets see what the other Agoners have to say.