Using Mark Levinson No. 380 preamp output simultaneously

If you are using both outputs to drive a bi-amps system, the gain level between the single ended and XLR output will not be matched

It's not a good idea because it unbalances the balanced signal.  The single ended output is taken from the non-inverting balanced output.  If the two phases of the balanced output see different loads they are no longer balanced.

Thanks for all the help..
@willcycle why is it not ideal? I was actually  thinking  about running the balanced to my mids and highs, and use the rca for my woofers (biamp) since my amp for my woofers only has RCAs

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.

I have a 380s.....I've used both outputs to biamp my Infinity RS-1s for 20 years or so.

I have a Levinson Reference No. 32 preamp (no longer in use, loaned to a friend).  I use to use two outputs--one to the main two-channel amplifiers, the other went to an ambient sound synthesizer that fed four smaller speakers set up to the side and rear of the room.  

That is a less common use than for subwoofers.  Most linestages can accommodate two outputs--whether by having two sets of jacks or use of a splitter--and there should be no problems with significant sonic degradation.

Why would you do this?

Some subwoofers require a line level signal from the preamp. 

n80"Please explain bi-amping in this context."

Biamplification requires the installation, implementation, and use of two distinct, separate, individual stereo amplIfiers or alternatively the use of four active stages of amplification that could be achieved with the use of multiple monophonic amplifiers or combination of such amplifiers with stereo amplifiers and in such implementations it could very likely be necessary, required, or mandatory to have two pairs of stereo outputs from the preamplifier used in the Music Reproduction System.

My question was not rhetorical. I'm new at this and still don't understand.

Please explain bi-amping in this context. 

n80"Why would you do this?"

It would be required, necessary, and vital to proper Music Reproduction System biamplification.

Why would you do this?

YES! No problem!