Terraform - Google cloud - create multiple service accounts with multiple roles

Question

I'd like to grant multiple Bigquery user roles to multiple service accounts. With for_each in Terraform, I can only use 1 variable. How can I use 2 for_each in one resource. Below is the code I tried:

main.tf has below:

resource "google_project_iam_member" "bq_access" {
  for_each = toset(var.bigquery_user_role_list)
  project = var.project
  role = [ for role in var.bq_roles : "${role}" ]
  member  = each.value
}

variables.tf has below:

variable "bigquery_user_role_list" {}

  variable "bq_roles" {
    type = set(string)
    default = ["roles/bigquery.viewer",
               "roles/bigquery.reader"
    ]
  } 

Error:

role = [ for role in var.bq_roles : "${role}" ]

var.bq_roles is set of string with 2 elements

Inappropriate value for attribute "role": string required.