When I applied to Stanford I submitted an arts supplement for music as well. By contrasting pieces they almost certainly mean pieces of different musical eras and styles. For example, some Bach and some Chopin.
I think you should be careful not to pick a lyrical piece that is too easy. They both should be difficult pieces, although one can sound more “lyrical” and one more “technical” (I don’t like using those terms, even the most “technical” etudes can be extremely poetic).
I think it is not extremely important, as long as your pieces are not all by the same composers. I think its fine if the pieces are from the same era. “Contrasting pieces” could also mean different types of pieces, for example a movement from a Bach suite and then a sonatina. Regardless, you will be fine as long as there is some distinction between the pieces.
Good luck! One word of advice: its better to play technically easier pieces really well then it is to play more technically impressive pieces worse. Make music! don’t try and show off