Is it permissible to combine two vows in one sacrifice, the first for success in studies and the second for buying a car, along with feeding a group of people in both cases?

If the first vow was conditional on passing the first-semester exam, then it is not obligatory to fulfill it because the condition upon which it was suspended has not been met. If it was not conditional, then the one who made the vow must fulfill it. Similarly, the second vow must also be fulfilled, as both are independent vows, and one does not suffice for the other.