Usually when people talking about having too many security apps and them interfering with each, they refer the conflicting of two programs
real-time protection.
QUOTE
This monitors computer systems for suspicious activity such as computer viruses , spyware, adware, and other malicious objects in 'real-time', in other words while the data is coming into the computer (for example when inserting a CD, opening an email, or surfing the web). Usually, this is done by analysing the data before it enters the computer. In fact, it also means the analysis of all data in files that the user attempts to access, even if these files are already on the computer. This means analysis of such a file before it is loaded into the computer's active memory.[1] It is used in contrast to the expression "on-demad scan" or similar expressions that mean a scan of part or all of a computer that is activated by the user http://en.wikipedia.org/wiki/Real-time_protectionUsually having two programs with realtime protection means they fight for control of the system, therefore usually slowing it down and often causing crashes.
Windows defender contains
real-time protection. Here is an example of
real-time protection used in
Windows Defender.

I think
McAfee Security Center uses real-time protection, but I am not sure. Despite this they may not conflict with each other. If your computer is running fine at the moment and both programs do not 'hog' too much of your resources I would say most probably not interfering with each other.