They can be split at the beginning of the game or at the beginning of every turn. I'm not sure about the full rules about splitting them up, but on the basic cards you can only split guys by preset teams. Usually that means a common model, a rifleman, and a special weapon going with each team.
No word on how to join them back up together. Although, it's probably the same as declaring it at the beginning of a turn and making sure the unit is in coherency.
The whole splitting up the squad is a nice dynamic, although you can cover more area, individual teams are more susceptable to suppression.