やりかた

providerにaliasを設定する
各リソースでproviderを指定する
providerを２つ以上定義する場合、全リソースにどのproviderを利用するかを書かないといけないっぽい
CloudFront、TrustedAdvisorなどus-east-1リージョンでしか管理されていないリソースがいくつかあって、それを他リソースと混じったリポジトリで管理していく場合に必要っぽい

ソース

main.tf

provider "aws" {
  alias   = "us"
  version = "~> 2.9"
  region  = "us-east-1"
}

provider "aws" {
  alias   = "japan"
  version = "~> 2.9"
  region  = "ap-northeast-1"
}

vpc.tf

resource "aws_vpc" "japan" {
  provider   = aws.japan
  cidr_block = "172.16.0.0/16"
}

resource "aws_vpc" "us" {
  provider   = aws.us
  cidr_block = "192.168.0.0/24"
}

terraform plan

------------------------------------------------------------------------

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_vpc.japan will be created
  + resource "aws_vpc" "japan" {
      + arn                              = (known after apply)
      + assign_generated_ipv6_cidr_block = false
      + cidr_block                       = "172.16.0.0/16"
      + default_network_acl_id           = (known after apply)
      + default_route_table_id           = (known after apply)
      + default_security_group_id        = (known after apply)
      + dhcp_options_id                  = (known after apply)
      + enable_classiclink               = (known after apply)
      + enable_classiclink_dns_support   = (known after apply)
      + enable_dns_hostnames             = (known after apply)
      + enable_dns_support               = true
      + id                               = (known after apply)
      + instance_tenancy                 = "default"
      + ipv6_association_id              = (known after apply)
      + ipv6_cidr_block                  = (known after apply)
      + main_route_table_id              = (known after apply)
      + owner_id                         = (known after apply)
    }

  # aws_vpc.us will be created
  + resource "aws_vpc" "us" {
      + arn                              = (known after apply)
      + assign_generated_ipv6_cidr_block = false
      + cidr_block                       = "192.168.0.0/24"
      + default_network_acl_id           = (known after apply)
      + default_route_table_id           = (known after apply)
      + default_security_group_id        = (known after apply)
      + dhcp_options_id                  = (known after apply)
      + enable_classiclink               = (known after apply)
      + enable_classiclink_dns_support   = (known after apply)
      + enable_dns_hostnames             = (known after apply)
      + enable_dns_support               = true
      + id                               = (known after apply)
      + instance_tenancy                 = "default"
      + ipv6_association_id              = (known after apply)
      + ipv6_cidr_block                  = (known after apply)
      + main_route_table_id              = (known after apply)
      + owner_id                         = (known after apply)
    }

Plan: 2 to add, 0 to change, 0 to destroy.

------------------------------------------------------------------------