I would just hire a fabricator they would have the glue and the special clamps. Look on you tube and you'll see what I mean. Also the cabinets need to be bolted together, not just placed together-but you probably already knew that.

Run a piece of masking tape down each side of the top of the seams. Buy some epoxy at a big box get the thickest consistency you can. Pull the cabinets apart 6" or so. Using a straightedge, make sure the pieces are on plane to each other. After cleaning the edges with acetone, mask off the bottom and edges of one side of the stone. Apply the epoxy, push the pieces together, and tape the bottom so the epoxy doesn't run out. You want epoxy squeezing out the top. If it leaks, apply more when the first round has set up. When you've got continuous set-up squeeze-out, pull all the tape and scrape the excess to flush with a razor blade.