Are Reserved Instance (RI) benefits aggregated across instances?

According to Reserved Instances - How You Are Billed the RI benefit is applied to 3600 seconds of running time across all your instances in a clock-hour.

In your case if you're running the instances concurrently from 10am to 8pm the RI benefit will only offset one instance, however if you run one instance 10am to 8pm and another one from 8pm to 10am it will be applied to both. Even if you run one instance from 10:00 to 10:15 and the other instance from 10:00 to 10:45 - that is in that clock-hour they will run for 3600 together, even if overlapping for some time, you the RI discount will apply.

Have a look at that link Reserved Instances - How You Are Billed - it's well explained there.

Hope that helps :)

You'll have to pay the on-demand rate for one of the instances.

You can run multiple instances concurrently, but can only receive the benefit of the Reserved Instance discount for a total of 3600 seconds per clock-hour; instance usage that exceeds 3600 seconds in a clock-hour is billed at the On-Demand rate.

For example, if you purchase one m4.xlarge Reserved Instance and run four m4.xlarge instances concurrently for one hour, one instance is charged at one hour of Reserved Instance usage and the other three instances are charged at three hours of On-Demand usage.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts-reserved-instances-application.html

Which specific instance will receive the benefit of the reserved instance pricing during any given hour is undefined, but this has no impact on what you ultimately pay.