example.tf

# Reference https://www.terraform.io/docs/providers/aws/
# Reference https://github.com/hashicorp/terraform/pull/8506

provider "aws" {
  assume_role {
    role_arn = "arn:aws:iam::${var.account_a_id}:role/${var.role}"
  }
  name = "accountA"
  profile = "${var.role}"
}

provider "aws" {
  assume_role {
    role_arn = "arn:aws:iam::${var.account_b_id}:role/${var.role}"
  }
  name = "accountB"
  profile = "${var.role}"
}

resource "aws_iam_role" "testa" {
  provider = "aws.accountA"
  name               = "test"
  assume_role_policy = "${file("policies/AssumeRoleLambda.json")}"
}

resource "aws_iam_role" "testb" {
  provider = "aws.accountB"
  name               = "test"
  assume_role_policy = "${file("policies/AssumeRoleLambda.json")}"
}