I'm afraid I have to disagree.
B30.8 A Pillbox counter in the same hex with a trench and/or in a hex Accessible to a Trench counter’s hex is considered a bunker. A bunker is treated as a pillbox in all respects except that a unit may move/rout/advance/Withdraw-from-CC between a bunker and such a trench as if the bunker were also a trench...
The first sentence specifies when a pillbox becomes a "bunker". It requires a trench in the same or an Accessible hex. The second sentence says a unit may move etc. from a bunker to "such a trench". "
Such a trench" means the trench(es) that made the pillbox a bunker. Imagine a pillbox in 1A1, a trench in B1, and another pillbox in A2. The trench makes both pillboxes bunkers. A unit in bunker A1 may move etc. to the trench in B1 -- that trench is "such a trench", a trench that makes the pillbox a bunker -- as if the bunker was a trench. A unit in bunker A1 cannot move to bunker A2 as if the bunker were a trench because the pillbox is not a trench. The pillbox in A2 did not make the pillbox in A1 a bunker. I don't think anything would change if the trench were in A1 instead of B1; when moving from one bunker, the other bunker is not "such a trench" that made the pillbox a bunker.
A bunker being considered the same as a trench for some purposes does not mean it is considered the same as a trench for other purposes. Otherwise if there were one trench in 1A1, and pillboxes in A2, A3, and A4 by chain reaction all the pillboxes would be bunkers: A1 is a bunker, consider it a trench, A2 is a bunker, consider that a trench, A3 is a bunker. This is obviously wrong, so a bunker is not considered a trench for making another pillbox a bunker, so it cannot be "such a trench" for the special movement ability.
So the unit would have to move from one bunker to the trench (1MF), survive D1F, then could move into the other bunker (2MF total).