Some difficult digging produced this thread. To summarize arguments:
Divine Challenge pertinent text:
On your turn, you must engage the target you challenged or challenge a different target. To engage the target, you must either attack it or end your turn adjacent to it. If none of these events occur by the end of your turn, the marked condition ends and you can’t use divine challenge on your next turn.
Given that the rules have no provision for any other way of extending marking and that neither power provides for mark extensions, and that, unusually, epic paladin does not have a multi-mark feat, it would suggest that only those conditions hold. However, maintaining mark on one target would count, functionally as "challenging a different target" for purposes of the second sentence from perspective of the originally challenged target, and thus, you would not be invoking a penalty if you failed to maintain /both/ targets.
I can get confirmation from Cust Serv if you'd like additional logic, however.
Additional interesting discussion, who is damaged when one person violates mark. This comes under the heading of "don't even try to ask your GM?" But is an interesting discussion.