I did said that you should search before writing up a question...
Since I'm answering (instead of commenting) I have a little more freedom in writing. So, analyzing your questions:
1. Is it a bad idea to run more than one antivirus on the same PC?
It is. No antivirus suite knows about each other running at background. So, your system will have an excessive overburden. Consider the following: you have two antivirus running full scans on your computer. What accesses what? And when? And how? Since two programs are doing intensive CPU and I/O processing, your system performance will plummet.
Besides that, most antivirus suites (giving AVG as an example) have some sort of a Resident Shield, that before any program running tries to quickly scan the file for potential hazards. Now if you have two antivirus suites, how are they going to deal that? Should suite A give priority to suite B, should the opposite happen, or should both try to open the file at the same time?
Probably the last on will be the one that happens most. Since no antivirus did read the file fully, they couldn't detect any hazard.
To sum up: Two or more antivirus suites = Less performance, more resource consumption, less security.
2. How far should one go to secure his PC? Where is the line between security and being paranoid?
I refer you to this question. Securing a new computer/laptop? [closed].
Essentially you can do what @terdon said and lock your computer behind a concrete bunker, never connected to power. And again I say, there is a lot of info about security in SU, just need to look it up.
3. Is trusting one Antivirus a good idea? I use two of the most well known antivirus, can't trust any.
Like I said on 1. having two antivirus suites is wrong, so, your experience is tainted by the fact you haven't left them running as they should: as only one running. So, try to uninstall one and see if you trust it, if not change.
You did said they detected different amounts of harmful data. Now, most modern antivirus suites use something called holistic detection. Has you might have figured out, no antivirus has copies of actual virus in their databases. They instead resort to certain behavioral rules that allow first to analyze an executable, figure what they want to do or how they were made and then determine if they are harmful. This leads however to the existence of false positives, that is, programs that, at the eyes of the scanner, seem harmful but are not, maybe because they were packed or compiled in ways the scanner doesn't know.
Now, you shouldn't trust on having a single protection. On my PC I have AVG as an antivirus suite, Malwarebytes as a malware scanner and Spybot as a spyware scanner. Since only AVG runs constantly (although Spybot has TeaTimer, that also runs on background), none of the other applications interferes with AVG detection.