The only way this works - and it's still not kosher - is if father and son have the same names. Just don't indicate junior or senior. Then you can potentially merge accounts. Not worth the hassle/risk.
I do wish US airlines had household accounts a la BA, but they don't, so I make do.
Remember that CO miles can be moved over to UA for free online now and you can easily book one way awards through UA. Simply book one from account A, one from account B for a round trip.