using two outputs from a dac?

Both will be live.  No issues so long as both devices have relatively high input impedance. 25K or higher for each should be more than adequate.

Thanks, Erik. I'm afraid I'm pretty dense about specs. The (tube) preamp input impedance is 50k but the headphone amp is 10k. Is that a problem? I should add that I'm not trying to run them at the same time (obviously?!). I should also say that this isn't a permanent set-up; the headphone amp will get its own dedicated system soon enough. 

It wont hurt anything to try but you may experience tonal changes and a loss if high frequency

When I purchased my Denafrips Pontus II DAC, I was informed by Alvin at Vinshine Audio that using both the XLR and RCA outputs simultaneously would result in a degradation of performance for the DAC. I also wanted to connect my DAC to both my integrated amp and headphone amp. Alvin then sent me a design for a custom cable that would solve this problem. The cable connected to the XLR outputs from the DAC and split off into two separate RCA connections to each of the amplifiers. I sent this design to Morrow Audio and they built the cable for me which has performed flawlessly. The only downside is having to use RCA instead of XLR to my integrated amp which I use way more than my headphone amp.

Thank you both so much. @erik_squires--that's reassuring, at least in the sense that I'm not going to fry (or blow up!) anything. @mksun, very bright and pretty cool to have such a bespoke cable. If this setup becomes permanent, I'll consider one for myself.

Usually all output are live unless your piece of equipment has output selector switch. 

It’s best to contact Bryson for their recommendation. It all depends on the output stage design. Some manufacture design separate output circuitry for single end and balanced output. Henceforth both can be used simultaneously. Other simply tap the positive leg on the balance output and therefore doesn’t recommend using both together.

just my 2 cents.

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.

Thanks, @desktopguy. I'm going to follow up and get the manufacturer's recommendation. I appreciate the advice!

Update: Bryston says it's fine to run them both. Thanks again for all the help!