Raj and neha drink water
Why it's not " ho"??
हो is only used when the subject is तुम, for all plurals you use हैं.
Why is "drink" conjugated as female? What is the rule if the subjects are male and female
Drink is not conjugated as female here, the female conjugation would be पीती. In this sentence there are multiple subjects, one is male and one is female, so the verb is conjugated to the neutral plural पीते.
If the subject was either one OR multiple females, then it would be पीती. A group of people with mixed genders OR a group of all males will be पीते. And पीता is used if the subject is a single male.