using two outputs from a dac?

Provided that you don't have a turntable, if your DAC has an analogue volume control, as does Audio Analogue's DAC, you don't need a preamp. 

I stream from my Aurender A30 to both my Luxman preamp and my Pass Labs headphone amp with no issues.  I do have whichever system I am not using switched to a different input. I can’t notice any degradation in performance. Aurender did confirm this setup was acceptable. 

Thanks, all. My DAC doesn't have a volume control but I can certainly turn off my preamp or switch the input on it.

Thanks again!

I had a Tisbury passive preamp (= RCA switch) for that purpose. Typically you don't listen to speakers and headphones at the same time so an A/B switch will work fine without worrying about impedances.

I have 4 DACs here. One has a single output while the others have 2 sets of outputs. Of those, two DACs are single-ended (both outputs are SE); and the third is a balanced DAC that offers both an XLR and SE output. Here’s what I’ve learned:

1 - If the manufacturer approves of using both outputs simultaneously, then by all means try it and see what you think. In my 3 DACs that allow this, I’ve tried each one on single output vs two outputs, and can’t hear any differences

2 - The convenience of dual simultaneous outputs can’t be overstated. I have two systems in this home office, and both DACs currently in use permit this. So in the main system, I can listen to headphones on one amp vs speakers/sub fed signal by he other output; or in the side system, I can listen to 2 headphone amps powered by the same DAC. This makes sonic comparisons pretty easy

3 - Finally, if the DACs manufacturer does not recommend using both outputs simultaneously, don’t bother trying it. They know what they’re talking about. I recently reviewed a DAC of this type (posted on Head-Fi), and the manual straight up advised against using both outputs.