I just realized this too.
There was not so much as a hint anywhere that the two headphone ouputs just as well could have been soldered to the main out. Or maybe they are. This is a dealbraker to me and the disappointment that this is not possible on this interface while apparently available on other (prior?) models was a huge surprise.
You just can’t blame the latency for this. This feature was expected. Even your manual made it appear that this was possible while misleading showing a configuration of two cue mixes in Studio One, named as Phones 1 and Phones 2 while it is still not possible to make that cue actually appear in the headphones on any other channel than Main.
Since buying other models will likely lead to similar suprises I’ll basically have to back out and return the product and get another brand.
Please, PreSonus, let us know if this is a UC problem or if it is hardwired!!