STL mid-month and 200DB

Hi, We need certain assets in our tax book that use the STL depreciation method to take 50% depreciation the first and list period.  We defined a calendar with 24 periods and this is working correctly for the categories that use this method.  We have other categories that require 200DB; however this doesn't seem to be available for this calendar.  Is that the case?  If so, how do you get around it?  Thanks much!