How to access multiple roles from single IAM user simultaneously?

Technically, you can assume multiple IAM roles at the same time but the permissions will not be aggregated. Assuming an IAM role doesn't change who you are or what permissions you have.

When you assume a role, you are given a new set of temporary credentials to use, instead of "your" credentials -- the credentials you used to assume the role.

When interacting with AWS resources each request can only be associated with a singular principal. So, if you assume role1 and role2 you can make requests as role1 OR role2 but not as both together. So, if you are trying to perform a single action that requires an aggregate of the permissions of multiple roles, that's a not possible.

Another approach could involve session policies. See: Create fine-grained session permissions using IAM managed policies.