Where to seam concrete countertop?
you dont need to seam it anywhere.  just pour it in one shot, and make sure you have steel reinforcing mesh and the glass fibre additive in your concrete.  its only 12 s 5 total size, so its an easy lift for 6 men.  this way you dont need to worry about fit up or future cracking.

this is one of the reasons concrete countertops are used.  they are no where near as durable as granite or silestone, but they don't need seams.