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.