Currently it seems it's not possible for a new build to be started unless the previous build has finished. If most of the builders are sitting idle it would be useful if we had the ability to start another build without waiting. (This may be mostly resolved by making ab01 not be special, which is in progress.)
This should actually be resolved now that ab01 isn't special. Test: Run two nightlies. Nightly 2 shouldn't be blocked by nightly 1.