why does running 2 anti-virus programs cause conflict?

In order to perform a real-time scan, the antivirus software must read the file as it is read from the hard disk.

Two antivirus programs trying to do so causes a conflict, as both programs insist on being the there first.

On a past computer, I wanted to replace one anti-virus for another. My first anti-virus program refused to let me download the second program. I had to completely uninstall the first program so I could download the second. Think of that conundrum ... I had to go online without an antivirus software so I could download it. The first program was so ungrateful!

I can imagine that it's a fail safe mechanism. Each program doesn't recognize the other as an antivirus program.

As an antivirus program, one of its duties is to try to prevent hackers from placing malware or other malicious programs onto your computer. Maybe each antivirus program sees its competitor as unidentifiable malware?

That sounds reasonable! they're probably see each other as 'virus'

I run Agnitum Outpost Security suite and Microsoft Security Essentials at the same time.
Both are anti-virus and are never turned off.
I have never had any problems.
I have also run Hitman at the same time (as required) with no problems.

Now firewalls are a different kettle of fish - one at a time is essential I think.

The conflict is confined to two running in real time concurrently; it is possible to have two AVs on your hard drive, with only one providing real-time protection, but then one has to be careful because the run running may want to delete or quarantine the other's definition files.