This is now venturing off into DIY, but it's unclear to me why exactly a 2nd stage regulator HAS to be in your mouth. Put it behind your head on a single snorkel tube, with a pushbutton purge cable to the back attached to the tube, and adjust the valve sensitivity to work in the new location.
(Yes, I'm aware of dead space in snorkels. The distance does not appear to be much greater going above the head vs behind.)
Though as far as I can determine, the real problem with long exhaust tubes is NOT the tube length, but rather that there is no way for water pressure to interact with the exhaust valve to keep breathing difficulty unchanged.
One option is an inductive design using an the exhaust as a venturi tube. This slopes upward behind the head, and yet keeps the exhaust opening at nearly equal water pressure. The bubbles will mostly tend to follow the open channels if the diver's head is in the normal level or down or sideways position, and breathing out induces a flow to pump water through the open front inlets.
(Yay, Windows Paint!)
Difficulty: There is feedback from water flowing past the venturi openings, which may trigger freeflow and require a somewhat less sensitive exhaust. Or just don't induce/venturi at all and use a straight T connection to the side tubes, using an upward slope from chin to ears, to direct bubbles up the tubes behind their head..
Another solution is to use a flexible exhaust valve at the regulator with direct water exposure. When open, the air does not release at the valve location but must travel up two balanced and connected exhaust tubes. Meanwhile water equalizing inlet valves are used to balance the exhaust suction as bubbles rush up the tubes behind the diver's head after the diver has stopped exhaling.
Difficulty: this design will fill with air if the tube ends are pointed downward / upside down and make exhaling harder. This only occurs when the diver tilts their head way up or is lying on their back.
The first venturi / T-connection version is less complicated, doesn't cause problems upside down other than bubbling out by the diver's face, and only requires hanging two open-ended exhaust tubes off of the existing regulator technology.
Wow, yet another potentially profitable, patentable idea, which I am stupidly posting to the world on a global public discussion forum under a semi-anonymous handle. I am such a moron.
However I am also lower middle class and unconnected with any diving manufacturers to make such things a reality, and I also know that to make any sense, patents must be applied for individually in each country for it to be effective, so that means something like 50 patents in all the major 1st world countries, and so patents are really only workable for the wealthy, and basically a scam for lawyers to take money from poorer people...