From my understanding of the rules, if you declare a multiple attack against persons with Super Senses and a person with Impervious, assuming you hit both targets, the person with super senses would roll to see if they dodge the attack before you split the damage, but the impervious figure wouldn't roll until you assign damage.

Is this correct, or would both SS and IMPRV roll before assigning damage?

I believe you are correct. Super Senses is rolled before any damage rolls (like BCF) so it makes sense that it would happen before you divide the damage (the SS roll basically tells you if you really hit the figure or not, so if they make the SS roll you didn't hit them and therefore cannot assign damage out of your total). Impervious on the other hand only reduces damage. You determine damage dealt and then roll Impervious to see if the figure shrugs it off.

Thanks Mr. P. That was exactly the answer I was looking for.

Since MrPilkington only "believes" he is correct I'll second what he said. Super senses is before damage is assigned, impervious is after.