In terms of RL playtime I think dualboxing will be faster than alternating characters. It's takes quite a long time to accumulate rested exp so unless you stretch out leveling most of your grinding will be without rested exp anyways. It takes about 6.5 days to accumulate a full level of rested exp. So let's say you plan on leveling to 60 over the course of 1 month you'll be rested for a total of ~5 levels if you always log off inside an in. That means you will level roughly 8.3% faster per character.

Compared to dualboxing, you gain 1/2 the exp from grinding but should be killing mobs 2x as fast and recovering 2x as fast (because you'll take 1/2 dmg because everything dies faster). The upper limit of dualboxing is probably leveling both characters at the same pace as solo leveling. So your total time leveling both characters could be as low as 1/2 the time. This is probably true for slow leveling classes that benefit from group dynamics like warriors but probably doesn't hold for faster classes (Hunter).

Personally, I'm dualboxing warrior-mage because I want to main warrior and thought mage would be a good complement during leveling and at 60 for farming. I've done some experiments grinding at various levels and found the rates to be reasonable and faster than I would be able to achieve solo as a warrior. For example, while grouped, I can hit 30k exp/hr per character at lv35 and 25-30k at lv30. These rates are better than I was able to achieve with a solo warrior. I haven't tested solo mage. It's only a few data points but I wouldn't be surprised is dualboxing warrior mage to 60 is faster than solo leveling the warrior.