$ aws sts assume-role --role-arn arn:aws:iam::012345678901:role/accountb-role --role-session-name test

$ aws ec2 describe-instances --profile AccountB

'aws_access_key_id'

[default]
region = us-east-1

[profile AccountB]
role_arn = arn:aws:iam::012345678901:role/accountb-role

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    },
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::012345678900:role/accounta-role"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1490625590000",
            "Effect": "Allow",
            "Action": [
                "sts:AssumeRole"
            ],
            "Resource": [
                "arn:aws:iam::012345678901:role/accountb-role"
            ]
        }
    ]
}