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:
Image Source: Personal Archive
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.