Need To Configure Parallelism Per-Build

I’ve got a fairly complicated use case that I can’t figure out if it’s possible on CircleCI. We have a fairly large Ruby RSpec/Spinach test suite that we currently run across two containers (and it still takes 30 minutes to build). We’ve been developing the test infrastructure to run our existing feature tests against Browserstack Automate browsers instead of Poltergeist/PhantomJS. It’s much slower, so we would like to only run the Browserstack build (with much higher paralellism, 4-6 containers, and entirely different test commands) on pushes to our release or prerelease branch.

The part we can’t figure out is how to have 2x parallelism for most builds, but have 4x parallelism for builds on a specific branch. We can customize the build commands per-branch with circle.yml, can we configure the level of parallelism that way too? This would really help us for this specific use case.