Generally, you can't combine the outputs if they share a common return path (or ground).

Almost all ATX and BTX PSUs having multiple 12V outputs actually have a single 12V output. Here's an example:

[![enter image description here][1]][1]

As you can see, there's only one output and two filtered outputs are taken from this single output. I don't think your PSU has a different structure for different 12V outputs.

So the short answer is, no.

  [1]: https://i.sstatic.net/9EA4d.png