A Very Hard fall

Cannot be played unless Percy Jackson and Annabeth Chase are both on the field.

Once both commanders are on the field, playing this enchantment links their toughness as if it were one.
If Percy had +0/+4 and Annabeth had +0/+3, their total toughness would be +0/+7

In addition, any spell that would destroy, exile, or sacrifice one of the commanders does not take effect unless both can be taken off the field.