With avast + mbam you are totally protected, having more than 2 Antivirus or 2 firewall etc you will have LESS PROTECTION.
The risk is greater and I will tell you why. Usually security programs conflict with each other and the result is loss of functionality and you can easily predict what happens next.
We had a customer that uses 4 firewalls, 5 anti-virus software and 3 anti-spyware software. So, judging after you, this customer had a bullet-proof configuration. But you are wrong: he also had more that 10 malware active. QED & case closed.