Here is the power supply that you have. Antec VP450
• 450 watts of Continuous Power • ATX12V version 2.3 compliant
• Dual +12V rails ensure greater system stability
The spec says "DUAL 12V rails"
If as you suggest, the power supply has just Two yellow(12V) wires, then it should be blatantly obvious that they are "colour coded" differently to show you they belong to the same rail, they're part of the dual rail. And even if they were not "colour coded". If they were only two(as you suggest) then obviously those are the two mentioned in the Specification, which says 12V dual rail!
This tomshardware forum link mentions something on it, but this post on overclockers forum is far more comprehensive.
To quote from it
What is "multiple +12V rails", really?
In most cases, multiple +12V rails are actually just a single +12V source just split up
into multiple +12V outputs each with a limited output capability.
Ok... What's the bottom line?
The bottom line is, for 99% of the folks out there single vs. multiple
+12V rails is a NON ISSUE. It's something that has been hyped up by marketing folks on BOTH SIDES of the fence. Too often we see
mis-prioritized requests for PSU advice: Asking "what single +12V rail
PSU should I get" when the person isn't even running SLI! Unless
you're running a plethora of Peltiers in your machine, it should be a
non-issue assuming that the PSU has all of the connectors your machine
requires and there are no need for "splitters" (see Example 1 in the
previous bullet point).
That might not answer everything but it answers a bit.