Render VSTi to mono wav?

[sboy]

I want to render all my VSTi tracks to wav ready for mixing. I have SampleTank with a bass outputting on ST's internal audio outputs 3&4. It's in a rack as I have drums on 1&2. In Tracktion, I have to choose left output comes from:Bass and right output comes from:Bass which means I always end up with a stereo file when rendering. Is there a way to force the result to be mono? Or do I have to convert the stereo file to mono in a wave editor?
Point is I don't need a stereo file for bass nor drums.