An attempt at two network interfaces per EC2 node - results in an error

simple-2.yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: 'Test - simple 2 network interfaces'
Parameters:
  KeyName:
    Description: 'Key Pair name'
    Type: 'AWS::EC2::KeyPair::KeyName'
    Default: kman
Resources:
  VPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: '30.0.0.0/16'
      EnableDnsSupport: true
      EnableDnsHostnames: true
      Tags:
      - Key: Name
        Value: 'simple'
  SubnetPublic:
    Type: 'AWS::EC2::Subnet'
    Properties:
      AvailabilityZone: !Select [0, !GetAZs '']
      CidrBlock: '30.0.1.0/24'
      VpcId: !Ref VPC
      Tags:
      - Key: Name
        Value: 'simple - public'
  SubnetPrivate:
    Type: 'AWS::EC2::Subnet'
    Properties:
      AvailabilityZone: !Select [0, !GetAZs '']
      CidrBlock: '30.0.2.0/24'
      VpcId: !Ref VPC
      Tags:
      - Key: Name
        Value: 'simple - private'
  InternetGateway:
    Type: 'AWS::EC2::InternetGateway'
    Properties:
      Tags:
      - Key: Name
        Value: 'simple'
  VPCGatewayAttachment:
    Type: 'AWS::EC2::VPCGatewayAttachment'
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway
  RouteTablePublic:
    Type: 'AWS::EC2::RouteTable'
    Properties:
      VpcId: !Ref VPC
      Tags:
      - Key: Name
        Value: 'simple'
  RouteTableAssociation:
    Type: 'AWS::EC2::SubnetRouteTableAssociation'
    Properties:
      SubnetId: !Ref SubnetPublic
      RouteTableId: !Ref RouteTablePublic
  RouteToInternet:
    Type: 'AWS::EC2::Route'
    Properties:
      RouteTableId: !Ref RouteTablePublic
      DestinationCidrBlock: '0.0.0.0/0'
      GatewayId: !Ref InternetGateway
    DependsOn: VPCGatewayAttachment
  SecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: 'Allow SSH and ping'
      VpcId: !Ref VPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: 22
        ToPort: 22
        CidrIp: '0.0.0.0/0'
      - IpProtocol: icmp
        FromPort: -1
        ToPort: -1
        CidrIp: '0.0.0.0/0'
      Tags:
      - Key: Name
        Value: 'simple'
  Host1:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: 'ami-02541b8af977f6cdd' # Amazon Linux x86
      InstanceType: 't2.micro'
      KeyName: !Ref KeyName
      NetworkInterfaces:
      - SubnetId: !Ref SubnetPublic
        AssociatePublicIpAddress: true
        DeleteOnTermination: true
        DeviceIndex: '0'
        GroupSet:
        - !Ref SecurityGroup
      - SubnetId: !Ref SubnetPrivate
        AssociatePublicIpAddress: false
        DeleteOnTermination: true
        DeviceIndex: '1'
        GroupSet:
        - !Ref SecurityGroup
      Tags:
      - Key: Name
        Value: 'simple - host1'
  Host2:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: 'ami-02541b8af977f6cdd' # Amazon Linux x86
      InstanceType: 't2.micro'
      KeyName: !Ref KeyName
      NetworkInterfaces:
      - SubnetId: !Ref SubnetPublic
        AssociatePublicIpAddress: true
        DeleteOnTermination: true
        DeviceIndex: '0'
        GroupSet:
        - !Ref SecurityGroup
      - SubnetId: !Ref SubnetPrivate
        AssociatePublicIpAddress: false
        DeleteOnTermination: true
        DeviceIndex: '1'
        GroupSet:
        - !Ref SecurityGroup
      Tags:
      - Key: Name
        Value: 'simple - host2'