Please note that ISO 27001 and ISO 20000 have different objectives, and core requirements, so only one of them is not enough to fulfill the criteria for both certifications. However, they share many requirements, which makes implement them together easier.
Now, regarding the necessity, this only can be evaluated based on your organization’s strategies and objectives. For example, if your core business is related to the provision of IT services and you have a clear demand for information protection, then both certifications would help.
These articles will provide you a further explanation about ISO 27001 and ISO 20000 integration: