The rogue feat "Two-Fisted Shooter" (Martial Power, p.140) says:
You can treat the hand crossbow as an off-hand weapon, and you can reload it one-handed as a free action. When you score a critical hit and have a hand crossbow in your off hand, you can make a ranged basic attack with that weapon.
So, if your attack with the hand crossbow was the weapon you were using to attack with and you crit, could you use a ranged basic attack with the same weapon (free reload doesn't take any time)?
If not, it seems like you could still wield two hand crossbows to get the effect.
This should combine extremely nicely with ruthless hunter, which increases the weapon damage to 1d8 and makes it a high-crit weapon.
I was just rereading this and, if it's possible, I'm pretty sure there is nothing in the language disallowing you from chaining crits — if you get one on the basic attack, I can't see why you wouldn't get another: the trigger is, "When you score a critical hit".
By the way, I know none of this is intended and someday it will be modified to say "When you score a critical hit with your primary weapon", but as written, I don't see a problem with it.