Merging hpc coverage from multiple architectures

I have a program that does slightly different things on different OS/architectures. I'll use this example:

main = do
    output <- readProcess "uname" [] ""
    print $ case output of "Linux\n" -> "tux"; "Darwin\n" -> "apple"

I would like to check the total test coverage of this program on all architectures.

To check coverage on a single architecture, I can run stack test --coverage (natively on a macOS machine and on Linux via a Docker container). It gives me .tix files and HTML reports that correctly show uncovered portions:

The runs produce the following files (slightly edited for readability):

$ find .stack-work/install/*/*/*/hpc -type f -not -name \*.html
.../aarch64-osx/hash1/9.10.2/hpc/merge-cover/tests/tests.tix
.../aarch64-osx/hash1/9.10.2/hpc/merge-cover/tests/tests.tix.premunging
.../aarch64-osx/hash1/9.10.2/hpc/combined/all/all.tix
.../aarch64-osx/hash1/9.10.2/hpc/combined/custom/custom.tix
.../x86_64-linux-tinfo6/hash2/9.10.2/hpc/merge-cover/tests/tests.tix
.../x86_64-linux-tinfo6/hash2/9.10.2/hpc/merge-cover/tests/tests.tix.premunging
.../x86_64-linux-tinfo6/hash2/9.10.2/hpc/combined/all/all.tix

How do I produce a report that shows me which parts are never evaluated across both runs?

• stack hpc report .stack-work/install/*/*/*/hpc/combined/all/all.tix crashes: can not find mypackage-0.1.0.0-hash3-tests/Main (hash matches neither of the directory names above).
• hpc combine .stack-work/install/*/*/*/hpc/combined/all/all.tix > combined.tix produces a file containing only Tix [].

I need to combine these runs to output a single report to feed into a code coverage service so that I know if a later PR misses some code. How do I do this?

• I have the project set up with Stack. I don't mind switching to Cabal for this, though.
• I'm not advertising a particular code coverage service (search for "code coverage service" online) but I don't mind switching to a different one either.
  • The code coverage service I currently use allows me to upload several coverage reports in its own format. However, this format is line-based and the example line above gets a coverage of "1/2" even after the merging, which is incorrect.