Dualboxing is worth it if you can only stand doing things at your own pace, otherwise there arent many benefits over soling. Tri-boxing is amazing when you have friends online to do things together. Tri-boxing same spec dps is really easy to raid on as well. I find Tri-boxing to be the sweet spot where the benefits begin to really outway soloing. If you're set on dualboxing i'd suggest classes that can heal and tank. Druid is a great duo since it covers every spec and has stealth for skips.