This will depend on the purpose of the run. If you're talking easy recovery/maintenance runs, breaking it up should not be a problem (you're just training your body to handle the mileage and shouldn't be going at a tough clip anyway). On the other hand, a hard tempo run at a specified distance depends on the time you're at an elevated pace. As Zeke points out, splitting this one up defeats the purpose of reaching a threshold and keeping it there (as the training effects don't kick in immediately and you are getting less training out of each run). Your long runs, regardless of effort level, shouldn't be split as the whole point is to train the body to go for 2-3+ hours.