is it necessary to get antivirus software and a malware software separately



If you want to stay 100% clean, yes. A separate anti-malware suite(malwarebytes) that deals specifically with malware is better suited for the task. For antiviruses, this is just an afterthought.
 
Is it necessary? Not really..

Is it better? Yes.

A solid antivirus like AVG will also block a lot of malware, but it won't do it as effectively as a dedicated anti-malware program like Malwarebytes.

Personally, I keep an antivirus program installed, but then download Malwarebytes periodically (as its a tiny program) when I feel Im overdue a scan.
 
I usually double up with both MalwareBytes Anti Malware free and Avast Antivirus free. Just be careful with Avast, they will try to trick you into getting the premium version instead of the free version.