After you install any vendor’s anti-virus, Windows Defender is not disabled; it switches to the passive mode. This option is regulated by Windows; no anti-virus installed in your system (including Dr.Web) can affect Windows Defender's operation. This is one of Microsoft's requirements.
All versions of the Dr.Web anti-virus are tested by Microsoft. During the testing process, Microsoft specialists confirm whether an anti-virus meets all their requirements. Abnormal behaviour of Windows Defender may indicate operating system problems or that some third-party software you installed has compatibility issues with Dr.Web. In such situations, we recommend that you contact Doctor Web's technical support service for a thorough analysis of the situation.