how about slow-dry JB Weld? i've done it before on a cracked and shattered slate. i gooped it, clamped it and then sanded it smooth the next day. i was shocked when i sold that table and we moved the slate and my fix didn't break.
Once you get the slates to match up perfect, the last thing you want to do is pull them apart again just to apply a 2 part glue. As it is it can take a while just to get the slates set with a straight edge to maintain a flat surface before adding glue. Believe me when I say superglue like water, will seep through the tightest crack.