If then bets

[notnosmallfeet]

I play on bookmaker and tried to do a if then bet today. I thought the point of them was being able to bet more than you have as in:

If bengals -3
then detroit vs. vikings o 42

i could bet 6 on the bengals then 12 on the vikings.

however it says i cannot bet a larger amount on the child than the parent....am i not correct in the way these work?