Nothing has changed from DirectRT v2008 to v2010 in this respect--i.e., constraints on randomization such that you can control patterns like this. For some examples of discussion, see:
http://www.empirisoft.com/support/showthread.php?t=689
http://www.empirisoft.com/support/showthread.php?t=2469

Or search on "constraint" and/or "constraints"