If I'm understanding you correctly, TestSwarm handles this, actually. If you watch the video walkthrough you can see the tests being split apart and run against

If I'm understanding you correctly, TestSwarm handles this, actually. If you watch the video walkthrough you can see the tests being split apart and run against multiple browsers simultaneously. Not only does this allow the results to complete faster but it also helps to make error correction easier (only re-running a sub-section of tests to try and get a good response, rather than re-running the entire suite).