Multiple Internet Security/Antivirus Software

Does having two or more Internet Security/Antivirus software on the same computer create operating conflicts?  I have Norton/Symantec 360 (paid subscription) and SystemMechanic softwares loaded on one of my computers.  A computer repair technician said having two or more similar protection services creates conflicts with each other.  Is this true?  I was unaware that System Mechanics was an Internet Security & Antivirus software.  I thought it was only a PC systems tuneup/troubleshooting and repair software.  Any help with this is appreciated?  I'm running WinVista 64bit system on a HP tower.