Is it possible to trade between pokemon Leaf Green and Emerald.?

I answered a similar question to this before, so my answer will be almost exactly the same as that one. It is possible to Trade between Leaf Green and emerald, but not until after you've progressed through both games.

You'll need to get the National Pokédex in both games though. To get the national dex on Emerald, beat the elite four and Birch should give it to you. On Leaf green, you need to beat the elite four and have more than 50 Pokémon in your pokédex.

On Leaf green, you'll also need to talk to Celio and agree to help him find the Ruby and the Sapphire. The Ruby is on one island. Head to Mt. Ember and you should find a couple of Rocket grunts near a tunnel You'll learn a password and then have to fight them. Once you've beaten them, head into the cave and you'll find the Ruby at the end. You'll need strength and Rock Smash to get through. Once you have the Ruby, go give it to Celio.

The Sapphire is a little more complicated. Firstly, head to Four island and go into Icefall cave. In here, after some exploration, you'll find HM07. This contains Waterfall, which allows you to climb Waterfalls. It's also a powerful attack. Use waterfall to climb the waterfall in Icefall cave and you'll meet up with Loreli. Help her take out the Rocket grunts and you'll learn of the location of the Rocket warehouse.

You can't get into the Rocket warehouse yet so head to Six Island. In the southern part of Six Island, you will find the Dotten hole. The door is locked and there is Braille on the door. Use Cut in front of the door and it will open. Then enter Dotted hole and solve the pitfall maze. To solve this, translate the Braille and go down the appropriate hole. The Sapphire is at the bottom, but a scientist will pinch it from you. On the other hand, he will tell you the other password to get into the Rocket Warehouse.

Now you have both passwords, head to Five Island and work your way through Five Island meadow. The Rocket warehouse is at the end. The warehouse is basically a maze of moving floors which you have to solve. There are also a few Rocket grunts and some useful items here. Once you solve the maze, you'll meet up with a Rocket Executive. She's a very tough trainer but once you beat her, she'll set up the floors so you can leave and get back to her without going through the maze. Use this opportunity to go and heal at the Pokémon centre, then go back and fight the second rocket execcutive. He is tougher than the previous one, and has the vicious Houndoom on his team. Avoid Fire attacks with this thing, they will only power it up. Use strong physical attacks such as Earthquake or Brick Break to take it down. Once you've beaten him, move further up and you'll find the scientist who nicked the Sapphire from you. Beat him to get it back, then give it to Celio.

Once you've given him both the Ruby and the Sapphire, you should be able to trade between Leaf green and Emerald. Be sure that you've beaten the Elite Four and gotten the National dex from Professor Birch on Emerald first.

You should be able to, me and my freind did with ruby/green.

There is nothing that I know of that you need to have to trade, if you want to berry mash, you both need a powder jar.

EDIT, when my firend and I traded, it instantly gave us the national pokedex, and he wasn't anywhere near the completion of the game.