How to stack multiple environments/projects for a reproducibility repository?

General Usage
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3

Ah, do you mean one should be modifying the LOAD_PATH by setting JULIA_LOAD_PATH appropriately? E.g., with a directory structure as

myroot/
  project_A/
    Manifest.toml
    Project.toml
  project_B/
    Manifest.toml
    Project.toml`

I would first instantiate both projects individually by running

cd myroot
julia --project=project_A -e 'using Pkg; Pkg.instantiate()'
julia --project=project_B -e 'using Pkg; Pkg.instantiate()'

and then I could stack the environments by setting JULIA_LOAD_PATH, e.g., like

JULIA_LOAD_PATH="$(pwd)/project_A:$(pwd)/project_B" julia -e '...'

where project_A would now be the primary environment, and project_B the secondary. At least that’s what I pieced together from looking at Code Loading · The Julia Language and Constants · The Julia Language.

EDIT1: Simplified the JULIA_LOAD_PATH command.
EDIT2: Remove @stdlib as suggested by @fredrikekre
EDIT3: Accept this answer incorporating the help from @fredrikekre to make it easier for future solution seekers.