Here are some tips on joining two guide rails. First thing, you need two connectors to couple the rails. Also, the ends of your rails may not be square, so butting the rail ends will likely not make the rails perfectly straight. I leave a small gap between the two rails, about 1/16", to have some play for alignment.

Slide the two connectors into the rails.

I then flip the rails over, so the under side is facing up. Using a 6' level for alignment, tighten the connectors. Make sure you do not over tighten the setscrews, they will dimple the rails.

With the rails right side up, tighten the setscrews.

You can also use the factory edge of your sheet goods for alignment.

Check the rails once in a while during use to make sure they are still aligned. One last tip, you can store the connectors in the rails so they won't get lost. Now all of your cuts can be straight.