Actually, being able to plant a second bomb might not be a bad idea, but there obviously has to be drawbacks.
First: If you can plant 2 bombs, then I say the requirements to plant 2 would be that there needs to be one at EACH bomb site, or they won't activate.
Second: If only one of these 2 bombs is deactivated, the other is still active and must be defused or it will detonate, regardless of the other being active or not.