Futakoi:

Twins daughters of a veterinary doctor and in the story they help their father in the clinic. Nozomu meets them when he takes Billy to the vet clinic after he ate a fax addressed to Nozomu which got it stuck in his throat. [Wikipedia]

Futakoi Alternative:

Ui Chigusa and her twin sister Koi Chigusa run a veterinary clinic. They request the aid of the Futaba Detective Agency to track down a missing dog.

(Source: Wikipedia)