One of the best split damage characters is the ranger summoner. It is primarily physical damage, but if you use elemental arrows and summon the incarnate to do some sort of elemental damage, it can be a great magic damage dealer as well.
Coming from the other direction, you can turn your mage from magic dealer to decent physical damage as well. Stick a couple points into Necromancy and then get Mosquito Swarm and Infect. These two skill are scale off of the Int stat. Mosquito Swarm only has a 3 round cool down so it can be used quite a bit. You can pick up a Necro summons skills (Bloated Corpse and Bone Widow) to do some more physical damage. Another little trick is to put two points into Aerothurge. This allows you to get Teleport which does pretty good physical damage. Using all of this and you can set up a mage to do solid physical damage when needed. I use the above tricks especially if I am running a Battle Mage. With a Battle Mage, picking up Decaying Touch is great as it scales off of Int. Have some way to get off a Restoration in combination with the Decaying Touch and the physical damage can be impressive.
If you go with these two suggestions, your remaining two characters can be used to make the party primarily physical damage or primarily magic damage or go completely split damage.
Keep in mind that your physical damage dealers can use grenades to do magical damage in a pinch. So, if you go with the 2 above suggestions and then take 2 physical damage dealers, you can put out pretty decent magic damage when needed.
