Actually, DS and DD have always been in the same class from Kindergarten through 3rd grade. The only exception has been DS' subject acceleration in math starting mid-year 2nd grade. DD was glad to get rid of "smarty pants" and I think enjoyed math more once he was accelerated. They really didn't have a choice about being in the same class after Kindergarten because there is just one GT class and both kids were already designated as belonging in the subcategory of kids that must be in the GT class as oppose to just qualifying for the GT class.
Keeping Aleks (or another online supplement) and having her do a little bit after school starts is another option that I am considering. However, I don't normally do any after-schooling and both kids usually rush through homework in after-care so they would have the whole evening free to play.