In the case of the orbiting object the acceleration is not zero. The object is undergoing a continuous change in its direction, which is an acceleration.
In the case of an object right in between two black holes, those two gravitational forces could cancel out, but if we look at the details of the time dilation between two different points it depends on the difference in gravitational potential, so while the force cancels out, the time dilations would actually combine to get bigger.
The way to visualize it is with the common rubber sheet analogy for thinking about spacetime curved by objects. The gravitational force is represented by the slope of the sheet. The steeper the slope, the stronger the force. If you're a distant observer very far away from an object, the sheet is flat and horizontal.
Time dilation depends on the height of the rubber sheet though, not the slope. The lower the height, the slower a distant observer will see a clock on that object running. At a point midway between two black holes, the sheet may be horizontal, but it will still be at a lower level than a distant observer.
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