Last weekend was another double exp event in Runes of Magic. My character entered at 40 Druid/41 Rogue and came out the other side at 47 Druid/42 Rogue. I've learned several things from my trek into the 40's.
How to stop worrying and ignore the Rogue
Your two classes in ROM have separate exp bars, and it's your choice how you choose to deal with this. You can attempt to level each class separately, which becomes more and more grindy as content starts to get low at higher levels, or you can park one of the classes and use it to turn in daily quest items farmed up by the other.
The pairing I really enjoy in the game is the Druid/Rogue, which is a DPS caster with a focus on DOT's and a primary nuke spell fueled by the Rogue's energy bar (a relatively unusual mechanic in an MMO). By contrast, the Rogue/Druid side of the house was a very good combination at lower levels (it's a rogue that can heal itself) but has become more and more frustrating as my levels advance.
Rogue energy regenerates at a slower rate than you see in other games because players are intended to use attacks from their secondary class (i.e. Warrior rage, Warden or Knight mana-based melee, etc) to fill the gaps in their rotation. The Druid side of this particular pairing does not contribute any attacks that are useful in melee gear, so I find myself in combat taking hits and waiting for the energy bar to regenerate. Anyway, I looked ahead and the 45/45 Elite Skill for the Druid side is not a high priority and the 50/50 Elite is only usable in groups, so I decided to park the Rogue side and wait for daily quest exp to push that bar along.
Perhaps this will change when I can pick up my third class if I take something that plays well with the Rogue, perhaps not. If not, I'll get by somehow.
Cash or Grind
The other thing that's really noticeable is that with double exp, the amount of experience from doing regular quests (and dailies that are sitting next to regular quests) is enough to level a single class. At the normal exp rate, it looks like I'm either going to have to do extra grinding of daily quests or pay for exp potions.
I'm not necessarily opposed to paying - my guesstimate is that this will run me about a dollar a night. Then again, I'm also not necessarily opposed to grinding a bit. I'm not always in the mood to log in and slaughter 150 mobs in rapid succession, but some days that's the level of brainpower I'm willing to spend. Otherwise I might sign on for long enough to do 10 trivial dailies to collect my daily tokens at best (or not at all). Or I might just wait until the next bonus exp weekend.
In a system where I pay for the amount I play (if I pay at all), rather than a fixed monthly subscription, I'm okay with this. It'll be interesting to see if this whole excursion grinds to a halt as I get beyond the launch level cap and into content that was balanced around superior gear (which I am unlikely to either earn or pay for), but ROM remains a nice secondary MMO in the mean time.