Hey,

Sciences subjects are more or less inter-related together . Even in life science you study multiple field. Therefore that give  you sufficient knowledge to excel in any other biology subject.

Practically it would have been  good if you had Net Jrf from Zoology in order to become lecturer in Zoology. But that's not a necessity. It's nowhere written that you have to be Net Jrf in specific subject to become a lecturer in that. Every vacancy involves a screening process comprises of written test or interview or both. If you will clear that you are good to be appointed as the lecturer.

Plus your double MSc will also help you in scoring good marks as your knowledge would be massive.

Hope that helps.

Thank you