How to join melamine sheets for a concrete mould?
Any seam with a void in the mold will result in a raise bead on the concrete top which could be polished down after curing. 

I doubt 5 people are going to be sufficient to lift and move a counter of that size. 5 may be able to lift it but moving it through doors and setting it place will be much easier with more people. I think your weight est is a bit low.  

I have the book "Concrete Countertops" by Fu-Tung Cheng and they specify clearly how to make molds for tops that are in two parts to be joined at the installation process but they do not talk at all about a mold longer than 8'. I recommend you get it. You may want to consider doing a pour in place counter top instead or come up with a design that incorporates the seam into it.   


  Idea: If you fill the seam with something, a resin or epoxy? and sand flat you can minimize the void but you need to research how the filler will work with or react with concrete. You would absolutely need it to be flat or you will create a void in the top and it would be more work to fill the void than to polish a raised bead. ANYTHING IN THE MOLD WILL BE TRANSFER TO THE TOP.