1. The jury should not base their decision on their belief whether either side is lying.
2. The defence is expected to lie, and if the prosecution cannot prove that every single one of the defence's arguments are lies, then the jury cannot convict beyond reasonable doubt.
3. The jury should assume that the prosecution is lying by default, and acquit if the prosecution does not convince them otherwise.
The defence is expressly prohibited from lying. Lawyers have a so called duty of candor [1] outlining this. Defendants testify under oath to make this clear to them. Defence attorneys must disclose to the court if their client lies to the court (and they can't convince their client to voluntarily disclose it instead) [2].
That doesn't mean people are expected to take defendants at their word during trial, juries are allowed to decide they think that someone was lying, but they aren't expected to lie.
2. The defence is expected to lie, and if the prosecution cannot prove that every single one of the defence's arguments are lies, then the jury cannot convict beyond reasonable doubt.
3. The jury should assume that the prosecution is lying by default, and acquit if the prosecution does not convince them otherwise.