When you say "with Nuts! activation," what exactly do you mean? At least per Nuts FV versus Star Army 2nd Tour, the only difference I can think of textually is that Nuts specifies that groups must be activated in descending order of leader Rep while SA is explicit that any activatable groups can be activated in whatever order the player desires, but that difference doesn't apply here.
SA p17 is pretty explicit about resolution order:
ORDER OF FIRE RESOLUTION
Here's the easiest way to resolve fire between two groups.
* Designate all targets.
* Resolve all fire from all shooters.
* Roll for all damage to all targets hit.
* Resolve damage as needed.
* Eligible targets now can return fire.
* When all fire is resolved from both groups, Unit Casualty Tests are taken.
In your case, both those Rep 5 soldiers must've been part of the same group, otherwise they couldn't have fired in the same activation action. Given that, both shots were fired simultaneously, fire would be resolved for damage and since the single target was dead there would be no way for him to make any Reaction Test (being dead is the most powerful reaction you can make, after all).
Since the difference in Rep would only affect whether or not they hit their target, it would have no actual difference on the result in this case.
Remember: actions, then survivors react. All fire exchanges complete, then reactions go off. Given the lethality of 5150, this means there aren't usually a lot of reactions to deal with in short order.