1. Observer A and Observer B are both stationary relative to a black hole, a considerable distance away. They each have a stopwatch, and both stopwatches are synced at t=0.

2. Observer A begins freefalling towards the black hole, while Observer B remains stationary relative to the black hole.

3. Just before Observer A reaches the event horizon, he switches on his jetpack to hover above the event horizon, stationary relative to the black hole.

4. Now Observer B begins the same freefall that Observer A just did, until she is level with Observer A, at which point he switches off his jetpack and they are both inertial again. If needed to satisfy any sceptical parties, Observer B can switch on her jetpack until she is stationary relative to Observer A.

The question is, whose stopwatch is lagging behind? The answer will determine who has accelerated more (obviously they have both accelerated at some point).