The more people in the group the tougher it is, and since while boxing you're not 100% on the slave you will find it a little harder. Just like in WoW.

Dualboxing is great for farming. If the problem you have running solo is because of gear then boxing will work great for you. Your magic-find stat will stack with all members of the group. Max out your slaves with magic-find gear, and dont stress when they die.