There is published data suggesting that you can add the two areas together and ue a cut off of 11mm2 ( 0.11cm2)
But the article states it is not as good as assessment of a non bifid nerve.
In these cases I concentrate on the other components of the assessment (change in contour / step sign / fascicle oedema / crinkle cut chip sign / etc.)
Check for space occupying lesions, solar arthritis, tenosynovitis etc.
Remember when you have a bifid nerve it commonly has a persistent median artery accompanying it. (but not always). The PMA is usually between the 2 nerves or on the ulnar aspect of them. Check for PMA thrombosis as this may occur and create acute symptoms.