Is it possible to create secondary IPv6 addresses on an EC2 instance, similar to how it's done with IPv4 addresses?

Yes, multiple IPv6 addresses can be configured.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/MultipleIP.html#working-with-multiple-ipv6

For more information on how multiple IP addresses work, please refer to the following document.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/MultipleIP.html#MultipleIPReqs

Incidentally, to set up IPv6 on EC2, you must have an IPv6 CIDR block tied to the VPC.
https://docs.aws.amazon.com/ja_jp/AWSEC2/latest/UserGuide/using-instance-addressing.html#ipv6-addressing