Multiple user_data File use in Terraform

Did you try template_cloudinit_config?

Add below codes.

data "template_cloudinit_config" "master" {
  gzip          = true
  base64_encode = true

  # get common user_data
  part {
    filename     = "common.cfg"
    content_type = "text/part-handler"
    content      = "${data.template_file.userdata_common.rendered}"
  }

  # get master user_data
  part {
    filename     = "master.cfg"
    content_type = "text/part-handler"
    content      = "${data.template_file.userdata_master.rendered}"
  }

}

# sample code to use it.
resource "aws_instance" "web" {
  ami           = "ami-d05e75b8"
  instance_type = "t2.micro"
  user_data     = "${data.template_cloudinit_config.master.rendered}"
}

Let me know if it works.