I agree with the above suggestion. Mirror cutting will get you a perfect joint. The few times I had to use this method, I had good results by not moving the straight edge once set.
Fasten the straight edge to one slab and make a cut - you can use a factory edge on good plywood or any number of straight edges, just make sure there isn't a bow created in the middle of the straight edge caused by pressure of you pushing the saw through the cut. If you have to, back up the middle of the straight edge so you don't get a bow in the middle. no reason to push the saw to fast, just let it cut slowly. Butt the other slab against that new cut and cut again. If the joint isn't perfect, slide the slab up against the first slab and cut again.
I had to use this method with a very large timber frame beam saw with a 16" blade when I made a table top that was 5" thick. The 13' joints were perfect, but took some time to do.