## current configuration:
$ guix describe --format=channels
(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (branch "master")
        (commit
          "a0751e3250dfea7e52468c8090e18c3118d93a60")
        (introduction
          (make-channel-introduction
            "9edb3f66fd807b096b48283debdcddccfea34bad"
            (openpgp-fingerprint
              "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))

## add guix-hpc channel (for illustration purposes only; you can use any other channel):
$ cat channels.scm
(list
  (channel
    (name 'guix-hpc)
    (url "https://gitlab.inria.fr/guix-hpc/guix-hpc.git"))
  (channel
    (name 'guix)
    (url "https://git.savannah.gnu.org/git/guix.git")
    (branch "master")
    (commit
      "a0751e3250dfea7e52468c8090e18c3118d93a60")
    (introduction
      (make-channel-introduction
        "9edb3f66fd807b096b48283debdcddccfea34bad"
        (openpgp-fingerprint
          "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))

## both channels - guix and guix-hpc - are available:
$ guix time-machine --channels=channels.scm -- describe
  guix a0751e3
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: a0751e3250dfea7e52468c8090e18c3118d93a60
  guix-hpc 1a6af9f
    repository URL: https://gitlab.inria.fr/guix-hpc/guix-hpc.git
    branch: master
    commit: 1a6af9f0154f57dc5a31bf535bb09db63e3a8d03

## but repl does not see guix-hpc modules:
$ guix time-machine --channels=channels.scm -- repl
...
scheme@(guix-user)> (use-modules (airbus solvers))
While compiling expression:
no code for module (airbus solvers)
scheme@(guix-user)> %load-path
$1 = ("/gnu/store/dfm3k2qc0s4f8vi5il3gvrr0650p65k3-guix-module-union/share/guile/site/3.0" "/gnu/store/1jgcbdzx2ss6xv59w55g3kr3x4935dfb-guile-3.0.8/share/guile/3.0" "/gnu/store/1jgcbdzx2ss6xv59w55g3kr3x4935dfb-guile-3.0.8/share/guile/3.0" "/gnu/store/1jgcbdzx2ss6xv59w55g3kr3x4935dfb-guile-3.0.8/share/guile/site/3.0" "/gnu/store/1jgcbdzx2ss6xv59w55g3kr3x4935dfb-guile-3.0.8/share/guile/site" "/gnu/store/1jgcbdzx2ss6xv59w55g3kr3x4935dfb-guile-3.0.8/share/guile" "/home/user/.guix-profile/share/guile/site/3.0" "/home/user/.guix-profile/share/guile/site/3.0")