Hi varivera,
currently both the EyeX Engine and the EyeX Controller are limited to one tracker per machine. But the Gaze SDK doesn’t have this limitation, and the REX eye tracker doesn’t either. So if your application demands it, a Gaze SDK-with-double-REXes setup is what I would recommend. (Or one EyeX Controller and one REX.) This is clearly not as easy to work with as the EyeX SDK but it can be done.
Please check out the Gaze SDK developer’s guide, particularly the parts about display setup and calibration, for more information. It’s a pdf document included in the SDK package which can be found on the downloads page.
You will probably see some interference between the eye trackers leading to a percentage of the gaze data packets being lost. We have tried this and found the data loss to be acceptable. (Sorry, I don’t have any numbers I can share. It was too long ago.)