Leave 1/16" between the panels. If you try to go less than 1/16" you will end up with bubbles in the silicone.
You can even leave a little more than 1/16", Aric says.
You have to run blue tape down the glass on both sides so there would be four strips of blue tape. You want the blue tape to go up to the little 45 degree arrisses on the flat polish.
There are two ways to silicone the joint. If you end up with a little more than 1/16", you can cut a small tip on the silicone cartridge and squeeze silicone into the joint, making sure that the silicone squeezes out on the other side. You don't want voids.
Using the sleeve over top corner clamp (SOGC90) you can easily lean one panel out a bit to allow for easier squeezing of silicone into the joint. Then push them back together and tighten the top clamp, leaving 1/16" between panels. Don't forget the blue tape. Then run your finger down the inside and outside, pushing hard enough to get all the excess off the blue tape.
It is important to get the excess off the blue tape. It is best if you and a helper run your fingers at the same time so that it doesn't push more back out on the other side. If you leave some excess silicone on the blue tape edge, when you pull the tape off it will make a mess.