About your question of running two anti virus I would not.  Most of the time when you do they will not work together real well and will slow down your system.  I would run the one you just purchased and then I would download a couple like Malware bytes and spybot and then run them about once per week.  

There is no perfect anti virus that will stop everything.  The best thing is to get a good one and keep it updated.  Second thing is to watch computer use.  Even if you really try and avoid risky sites it still can happen by simply mistyping something in a search and getting sent to a bad site.  Sometimes even normal good sites can infect a PC.  I have cleaned a few now that came from something on Facebook.  

There are plenty of good anti virus out there and a number of them for free.  I used Norton for many years and never had a problem.  Lately it was running about $70 per year and that seemed a little high.  I have dropped it and now you Microsoft Security Essentials as the main and then use Spybot, Adaware and Malwarebytes as addition cleaners.  All of them are free and look for slightly different things.