This is a bit of a tough one, but if I had to decide - I'd go with the Ranger suffering -2 to hit the Warlock and would also incur the radiant damage. Specific > General.
Divine Challenge specifically states:
"... takes -2 penalty to ATTACK ROLLS for ANY ATTACK which does not include you as the target."
Did they make an attack roll against someone other than the Pally?
Did they "attack" anyone other than the Pally?
If you answered yes to either/both of those questions, then Divine Challenge kicks in, imo. If that isn't what they intended, they would have to SPECIFICALLY say so in the description/mechanics of Twin Strike.
*edit*
Lets also not forget... Twin Strike is technically a "Ranger Level 1 Attack POWER". If that power grants 2 different attack rolls and 2 different attack targets, then it provides 2 different attacks. Period.
Divine Challenge specifically states:
"... takes -2 penalty to ATTACK ROLLS for ANY ATTACK which does not include you as the target."
Did they make an attack roll against someone other than the Pally?
Did they "attack" anyone other than the Pally?
If you answered yes to either/both of those questions, then Divine Challenge kicks in, imo. If that isn't what they intended, they would have to SPECIFICALLY say so in the description/mechanics of Twin Strike.
*edit*
Lets also not forget... Twin Strike is technically a "Ranger Level 1 Attack POWER". If that power grants 2 different attack rolls and 2 different attack targets, then it provides 2 different attacks. Period.