I am 100% sure that you would not be able to switch to a double degree when you got an offer just for the engineering degree, even if you want to do it on the enrolment day, it would be too late (because it now involve two faculties instead of just 1 faculty).
However, there are 3 ways that you might be able to do it:
1) try to get a final round offer by putting B engineering/B commerce as your first preference in your UAC application form
2) apply for a mid-year entry through UAC, usually during june (only if the combined degree you want allow the mid-year entry). For mid-year entry, they only look at your UAI, not your tertiary grade for semester 1)
3) re-apply at end of the year through UAC for 2007 entry (the UAC may then offer you a place for 2007 by taking both your 2006 UAI and your 2006 tertiary grade into account)
Hope all these help!!