[Paul Maunder]

Sorry for misunderstanding your question. There is a way to do this, but it's a little bit fiddly:
First import the two mono audio files into your session.
Put them onto two mono audio tracks on the timeline, with both regions starting at the same location.
Ensure that the two mono tracks are centrally panned so you can audition what they'll sound like when combined together.
Next, use the trimmer if necessary to make the two files exactly the same length.
Rename one of them 'something.L' and the other one 'something.R'. This will make protools see them as a single stereo file rather than 2 mono ones.
Now select this 'stereo' file from the regions list and select 'export selected as files'.
By choosing to export it as mono, you'll combine the two, and much more quickly than bouncing to disk.
The only drawback of this is that you can't put in any automation data, and the relative level of the two files is fixed to whatever level they were recorded at. You could potentially change the level of one using the normalize plug-in to balance it out as needed before exporting. Anyway, I hope this helps.