Personally I'd always say start from scratch, mainly as I find I like levelling more than I enjoy end game. (I have a 70, in a good guild, I hardly play.)

1-20 gives you a feel for the characters and classes you've picked especially if this is your first foray into multiboxing. Most people learn to play solo, moving from a solo game to a 100% grouped game is not always that easy, even if you know the classes inside-out. Besides if you decide that X really sucks it doesn't take long to train a replacement or two.

One thing about doing it this way is that you get time to iron out your control, before you actually need to. Even if you stick to PvE on a PvE server you will encounter situations that mean some part of your party has to run away, being able to seamlessly switch control is a useful technique. Above and beyond that if you have the system working well you can also use it to do collection quests.

Collection quests are the bain of a multi-boxer, no doubt about it, but they still generate useful XP. And unless you can stay in instances from 10-70 (Alliance, no idea how high you need for RFC) you will need extra XP. If you have the control right you can switch mains as you fill each character up with the needed items, I'd suggest also using Lightheaded, Tomtom and a compatible quest log mod (I use nQuestlog). Then you actually get to see what other people have already found out about drop rates. (Comments come from WoWHead.) A quest announcer is handy to be able to see how many kills others need for kill quests.