The answer is No, you cannot have more than one tag for one vehicle.
If Yes:
People can take multiple tags and use it on other vehicles of bigger size
It may also be little messy for the premature fastag system to deal with multiple tags
Some of the crimes are also tracked via fastag, so multiple tags can open another gateway
Discounts will go away once the providers have enough customers base. IMO it is not a wise idea to take fastag in the name of utilizing offers.
For people who rarely use fastag, it doesn’t matter with discounts, if 50INR cashback comes, fine, even if not, the journey has to be taken. And for people who uses it frequently, the discounts may not matter again here, because that could be their need.