If you click and hold the mouse button to direct the characters, the lead one will stay in the lead. There is no option to adjust how they pick locations around a single left click, however.
There is no way to automatically have someone switch weapons / attack methods. If you were leading with the other character and had the mage set to aggressive (with a weapon as primary attack) you could order him to cast a spell and he would attack with the weapon after that. This is fine for an archer using occasional spells, but for a mage the easiest solution would probably be hotkeys to change primary skills.
