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