Create a custom sovereign, and go into documents/my games/legendaryheroes/units folder, and find the custom sovereign file. 

add these two lines into the file along with the other SelectedAbility** thingies. 

<SelectedAbilityBonusOption>Death1</SelectedAbilityBonusOption>
<SelectedAbilityBonusOption>Life1</SelectedAbilityBonusOption>

Just tested it myself, and seem to work fine... except for leveling up the magic..... 

<SelectedAbilityBonusOption>Life5</SelectedAbilityBonusOption> might be the only way if you want to wield both kinds of the same time. (death5 if you play kingdom) Depends on if you feel like it's cheating or not. 

 Okay made a workaround for kingdoms. 

https://dl.dropboxusercontent.com/u/95181126/Deathmagic_for_Kingdoms.zip (extract into mods folder)

Add this line into your custom sovereign giving him "kingdom" deathmagic:

<SelectedAbilityBonusOption>Death1_2</SelectedAbilityBonusOption>

If someone knows how to make it customizable in create sovereign screen, please add to it, or tell me how so we can do a proper mod out of this ^^ 

Edit: Fixed broken link, and fixed placement in skill tree, you should be able to have all magics now if you want to

I'd rather do it properly if I invest more time in the mod (life magic for empire is absent etc.) If you only want to experience the life/death combo, the current .xml file should do the trick. 

CoreAbilities.xml, copy Life-Life 5, and all life spells in CoreSpells.xml. You need to copy those and modify them to a new path like I did with death spells. It's a bit of an hassle, and not a quick fix sadly

The mod only works when <SelectedAbilityBonusOption>Death1_2</SelectedAbilityBonusOption> is put into your sovereign. It should do nothing without it, and you could even forget it existed

Savefile can't be "modded" after the fact, so a save has only old information. You need restart the game, and start a new game for new mods to "update" usually.