Here are the combined sentences using double comparatives:

1. The harder she studies, the better grades she will get.
2. The more you rest, the better you will feel.
3. The warmer the weather is, the better I feel.
4. The earlier we leave, the sooner we will arrive.
5. The more knowledge we have, the wiser we become.
6. The younger you are, the more easily you learn.
7. The more expensive the hotel is, the better the services are.
8. The more electricity you use, the higher your bill will be.
9. The more I thought about the plan, the less I liked it.
10. The older he is, the more slowly he learns.
11. The more she eats, the fatter she will become.
12. The older she is, the more beautiful she becomes.