Are your UVs mirrored and stacked at all? Or are they totally unique? If they're stacked or overlapping you'll need to move the "mirrored" UVs outside the 0-1 range (since that's all that gets baked). Just make sure to move them by exactly one UV unit so that their relative position in texture space is the same. That's my…
Hmm, i have reset XForm, throwed a Mesh select modifier to the stack and fliped/unified the normals ... and now its ok. This thing took my whole saturday :/ Anyway, thank you all for the replies!