I run 8, and the trick is to invite everyone to one big raid group, then disband the group and create two smaller groups. I just made a button for the /jamba-team disband command on my lead character, and two /invite macros for each respective team of 4. Works pretty well.

It seems to only split me up if an area is really busy. It's settled down now that the expansion has been out for a bit, so there's less people in one area. It used to do it to me a lot at the class halls, but not lately. Even in Dalaran, sometimes I'm all in one shard when I hearth.

You can sometimes use this to your advantage, if you are on PvP, if you are surrounded by the other faction on one shard, but not on another, choose the appropriate shard (less enemies if you just want to quest, more enemies if you want to do world pvp)