When you subscribe to become an Exceptional Friend, you get another candle. Does this only mean your total capacity has become 40 (in other words, your one candle doubled in length)? Or do you replenish two actions every 10 minutes?
It only seems to double your total amount. This means that you're effectively wasting fewer actions (because you can keep earning them every 10 minutes once you've reached 20).
If you manage to log on often enough to never go above 20 (which probably means you don't sleep enough ;-)), then having them would have no benefit.
I can confirm that the candles do not replenish simultanenously. The second candle doubles the total number of actions that you can accumulate, but does not increase the rate at which they accumulate.