Then should I write both of them. Or just choose one?
The trick is to write not about one of them, but about YOUR unique intentions and ideas. You have gained some knowledge and wisdom, and your wisdom (insight) leads you to want to master both of these disciplines. That means you should do the following things:
Discuss concepts in both fields, and perhaps cite some articles.
Discuss the school's program
Discuss your specific plans for your professional future.
Your interests and plans will show why both are necessary.
Seeing water turn to be steam and disappear, an ordinary child will say it is magic, but, this sparks off my thinking.
It
is magic!
I want to learn the hottest thing since the Big Bang, the coolest things in the universe, the simplest things in our life, and whatever about every other secret of physics.