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Our preference for workflow at the company is to have a single repository per project. Since we often have projects that go dormant for long periods, this keeps the version numbers (that are tied to SVN builds) more consistent to the end user when updates do occur.
Because of this, we usually like to have a single repository per project so commits to a different project will not cause a large jump in the revision of an older project when we go to update it. (IE Project 1 is at rev 3, we make 100 changes to Project 2, now next bug fix to Project 1 puts the rev at 103)
Your use-case makes sense but OnDemand does not support this. There are a number of technical limitations in the way of implementing this, and will not be pursued in favour or working towards better DVCS support.
The feature request is at https://studio.atlassian.com/browse/JST-1462