The problem is that tunnel entrances must be within 3 hexes one from another.
One could imagine 2 tunnels having each an entrance in the same hex.
A unit could move (during MPh) through one tunnel to the "common entrance hex" (emerging during APH) and, another MPh (or RtPh) using the other tunnel.
I don't see how it could abstain from coming out between the 2 tunnels.
Imagine somebody has enough points : what would stop him from creating a 30 hex tunnel? How would he count movement in the tunnel?
There were no cave complexes in Stalingrad, by the way. :wink: