Reprocessing one minifier's output with itself is probably fine, since you're not mixing and matching approaches to minification. It's likely that a minifier is consistent with how it minifies internally, and they simply don't do multiple passes.
Your time would be far better spent, though, making a pull request to properly implement multiple passes on your minifier of choice so that you can safely get that benefit. Or you could say "meh, I'm not losing sleep over the extra hundred bytes" and work on something more productive.
