You can run both simultanoeusly. But in my opinion no need.
If you are having Avira Free Version, MBAM is good Combo.
Avira Premuim has all the features of AM.
If you need Extra Protection, You can Keep both.
Malwarebytes is good...and so I thought. It seems like A-squared has a much better detection rate. It isn't the first time where malwarebytes missed really serious virus/trojans and A-squared picked it up.
A while back, I was impressed when I saw how many threats A-squared picked up. A little too impressed. No other scanner was picking up what A-squared was. So, I did a fresh windows install after a full format, installed A-squared, and ran a scan. It was still picking up 'threats.' It seems to throw in extra detections to make itself look a little better.
I'd have to say combofix has been the most effective. Malwarebytes catches most everything I've seen. However, there was one PC giving me some real fits. No matter what I did, no scanners were finding the infection. I ran superantispyware, malwarebytes, sophos, avast, avira, panda, spybot and ESET. None of them could find it.
Combofix was the only scanner that found and removed the rootkit. Combofix is a little different. It doesn't show you the results, and ask you what you want to do. It just does it. It's powerful, but effective.