I was thinking it can simultaneously bite and tail in snake form, making it nastier than a regular constrictor.
The Werebear seems a better model for that than a Wererat, which only gets a single attack in animal form.
To avoid further confusion, I'll better spell out all the options…
Multiattack. The weresnake makes two attacks. In snake form, it makes one bite attack and one tail attack. In humanoid form, it makes two scimitar attacks or has one blowgun attack and one scimitar attack. In hybrid form, the weresnake's first attack is with its bite, blowgun or scimitar, and its second attack is with its tail or scimitar.
That ought to do.