a good trick for doing this - if you can get your head around working from the underside of the tops - is this.
get some cheap hardboard at least as wide as your tops
cut a mitre on one and place it over the other in situ - then mark with a knife the score for the other.
this takes any wall unsquarenes into account and gives you a full size marking template. it doesn't need to be full length.
then write 'top' on the top surfaces of the template
turn your worktops over so you are marking the base and transfer the cut lines from the templates again with a knife (the side marked top faces the worktop, remember)
now the clever bit
with a circular saw set to 5mm less than the thickness of your worktops cut along the score marks -on the bottom of your worktops - making sure that you remove the scored line.
the idea is not to cut all the way through.
you should not see any cut from the top surface of your new worktops.
be careful not to break the thin piece that joins the two halves of your cut.
use a jigsaw with the finest blade you can find to cut through the 5mm joining skin, making sure that you leave the overhang on the worktop part that you will use.
do that to both worktops
put them together so that the fragile overhangs just about meet.
get a sheet of aluminium oxide production paper (carborundum will do too, sandpaper will take longer) fold it in half insert it between the gap and slide it back and forth. the overhang is so thin it will sand away quickly and you will be able to see what needs removing because it will be where the two touch.
it will not take long to get the two worktops married perfectly.
as the 5mm overhang is so shallow it will not become a weakness in your worksurface
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you are left with a slight groove underneath, deal with this in either of the following ways:
put some masking tape on your precious surface right up to the cut.
pull the two worktops apart and liberaly overfill the grooves with silicon, gripfill, hard as nails, sawdust mixed with pva, anything that sets hard-ish and fills space.
If you can find any the absolute best thing here is one of the new polyurethane foaming adhesives, they are rock hard (stronger than the wood itself), completely waterproof (even set underwater) and are easy to clean off with a stanley blade where they have not been under pressure.
push the two worktops back together in situ and clean off the ooze from the masking tape (but leave the tape itself until you are done).
better clean any ooze underneath off too
or
use a dispenser gun to inject a flexible filler in from underneath later. use masking tape on the surface anyway.
it is a good idea to use some fixings underneath, either drilled metal plates or thin ply wherever they will not get in the way.