You can use it but it is not ideal. The manual urges against it. A better solution is to use a splitter off the balanced output and an active balanced to single ended converter.
Using Mark Levinson No. 380 preamp output simultaneously
The preamp has 2 outputs, XLR and rca, can i use it simultaneously without damaging the preamp? I have 2 amps connected and im just using 1 amp at a time.. i was thinking running both... will it damage it?