Just grab the head out of your chest and equip it? I see no reason why you'd need a command to have insta-access to heads.
Suggestion denied.

Now for an off-topic rant:
WHY THE ESSAYS.

This entire suggestion can be completely written out in just one line.

"A command that allows you to equip a players head using /head [playername]. You can only equip a players head if you've obtained it before."

A suggestion should be straight to the point, simply giving an idea. Not an entire story.
Please stop writing essays in suggestions, you can do that in things like Slap Snap, where it's actually good to write a long text. Just stop doing it here -.-