i have an experiment between two separate deployments based off a feature flag. is it possible to run an experiment only within one value of that first flag, so i can isolate a test within a test?

Yes, this “test within a test” pattern is supported, but it’s done via prerequisite features rather than literally nesting rules. Configure the experiment to depend on the flag's value as a prereq, so it only evaluates for users who would be in that group. https://docs.growthbook.io/features/prerequisites