I've added Brett after a primary fermentation in a secondary vessel, but never tried adding a second sacc yeast. That time I didn't add any extra sugars, just whatever the ale yeast couldn't finish up got left behind for the brett. Although, I guess I did add some homemade candi sugar to a dubbel I did after a few days of brewing, I suppose that would be similar to moving to secondary. Both are pretty tasty! If you've got something in mind, give it a shot. If you're going to add a lot of sugars in secondary, make sure you've got a little bit of headspace so you don't overflow!