One changeset per issue is a general goal. So is the ability to review patches. Sometime people forget to add a News or Acks entry and have to followup with an addendum. (Mark's patch still lack that, for instance.) Sometimes a patch is so large that a reviewer asks or requires splitting. In this case, each patch is large enough and touches enough files (up to 10 I think) that two is plausible. What would not be appreciated by either a reviewer or commit-list recipients would be one patch file per patched file. That would be a patch bomb indeed ;-)