I have handled the trench meeting in the middle problem two ways. The easiest was for a trench that did not need inspection—I dug about 75% one way, put the PVC water pipe in, and back filled enough to be able to drive over. Turn the backhoe around, start at the other end, and you are able to just back off and over the trench when you reach the backfilled area.
The second method I used was to fill the trench with sections of rail road ties to ground level, cover with three quarter plywood, and drive off. This trench had to stay open for an electrical inspection.
My backhoe will lift the tractor up off of the extended backhoe feet so I can move the machine right or left with the hoe controls to stay centered on the painted dig stripe but only if the FEL is off the ground so that the unit can rotate on the front wheels. Accordingly, I don’t think I could “walk” the backhoe off of the trench like the big Case (and other) machines can. I also don’t know how anyway—I need to see it done--but would probably be afraid to try it.
Another solution mentioned somewhere in TBN was to did a perpendicular trench (in your case 15 feet wide) to connect the two trenches. This has always involved tearing up too much ground for me, although it would work just fine.
Good luck,
Bill Vorhies