Josh, check out

http://www.sfrscca.org/RoadRacing/Supps-Re...uppRegs2_10.pdf

Section B

Requirements for a "Dual (Same Driver) Entry at $150 (2006 Price)

"Must be entered and not withdrawn in the original class. A dual entry is defined as the same car and driver a second group at the same Regional........"

Your right though, that you could enter ITX only, but intent of ITX was for dual entry. Double the track time for less than 2x standard entries.