I've recently stumbled over a trouble in my home studio.... coming out of output 7 and 8 on the digi002 rack (1/4" jacks) I want to merge into a stereo headphone port on a breakout box. How is it wired? i tried to do both TS mono tip to the tip and sleeve on the headphones, but didn't get it.

I need to solder them together myself, and I have both mono and stereo jack available.

never tried to do it, but my understanding is that the tip of the H/P is common GND so you would connect the tip of each mono plug to the tip connector on the H/P socket, and the the sleeve of the mono plugs go to the other connectors on the socket to give one conection to sleeve and one to the ring.
This should work. Which side is right and which is left will depend how you wire it up.

Let me know how you go.

edit; Just re-read your post and think your saying you tried this, tho it seems that your H/P socket is not TRS?