The double (or in this case, triple) targeting mechanism is currently thought to cause coding problems, but that doesn't matter for the time being.
This is only beneficial over Protect Artifact in the case of Dimensional Shield, Wings, Sundial, Cloak, Unstable Gas, and, in rare cases, weapons.
The downsides are that it costs more, makes the protecting creatures highly vulnerable, and I assume that if both creatures are killed by CC (or PC), then the protection is gone.
Lowering the cost would be a start, but I really don't see this getting much usage. The slot-targeting mechanism is a neat idea though.