Just to make it sure that the first son is from his blood.
While he is already married with his granddaughter, he can still have concubines, and the first male offspring will become Noah's successor.
That's why he said "i bet everything on Noah". If he wanted to be sure to have connections to the next emperor, he could send his second grandaughter to another candidate, hoping that she would conceive the first son among the legal wife and concubines, yet he decided to bet everything on Noah