I’ve this structure:

|-- combine.nf
|-- first
|   |-- a.txt
|   |-- b.txt
|   `-- c.txt
|-- second
|   |-- A.txt
|   `-- B.txt

And combine.nf is

#!/usr/bin/env nextflow
process sayHello {
  input:
  path first
  path second

  output:
    stdout

  script:
    """
    echo 'Hello couple (${first}, ${second})' 
    """
}

workflow {
    def files_first = Channel.fromPath("first/*.txt")
    def files_second = Channel.fromPath("second/*.txt")
    sayHello(files_first, files_second) | view { it }
  }

The sayHello process is only called for two pairs (the size of the smallest directory in fact):
Hello couple (a.txt, A.txt)
Hello couple (b.txt, B.txt)

How to process all possible pairs? Thanks in advance

PS: this question is generic, in my case one of the directory contains only one file.