Run specific provisioners on specific builds (packer 1.6.6)

Hi,
I’m new to packer so probably I’m making some mistake but I cannot find it.
I’m trying to understand how I can run specific provisioners on specific builds.

I tried to use the “only” directive (as explained at provisioner - build - Blocks | Packer by HashiCorp) but the result is that no provisioner is executed.

My code looks almost like this:

source “amazon-ebs” “AMI-ubuntu1” {
communicator = “ssh”

}
source “amazon-ebs” “AMI-ubuntu2” {
communicator = “ssh”

}

build {
name = “Build-AMI-ubuntu1-and-2”

sources = [
“source.amazon-ebs.AMI-ubuntu1”,
“source.amazon-ebs.AMI-ubuntu2”
]

provisioner “shell” {

only = [“source.amazon-ebs.AMI-ubuntu1”]
inline = […]
}
provisioner “shell” {

only = [“source.amazon-ebs.AMI-ubuntu2”]
inline = […]
}
}

Any help would be appreciated
Kind regards

Stefano