Regarding the tite joint fasteners, do you recommend using them with or without glue? And how many would you use across a 24" span?
I would use a minimum of three...one near each edge and one in the middle. Four wouldn't hurt. If you are going to use a spacer as a decorative addition to the joint, gluing the splines in would allow the top to be sanded flat.
Just to join the separate tops with the ability to ever take them apart you would need no glue. To just join the tops you could use half laps, as suggested with glue. The seam and any grain differential would be obvious.