。
It's not that I like both of them.
Literal
Both-together liked [it's-not-the-case-that-わけではない].
~わけではない is a partial negation pattern: 'it's not the case that X.' Here it means 'I don't necessarily like both' — the speaker may like one but not the other. Crucially different from 両方とも好きではない ('I don't like either'), which would be flat total negation. This partial vs. total negation distinction is a key intermediate grammar point.