Usually when you have two brushes touching each other, it's usually best to make a very small gap between the two brushes. Another thing you can do is make one of the brushes a func_wall, but it doesn't cast a shadow when light hits it. One of the best things to do is to scale the textures in your map, the smaller they are scaled, the higher the wpoly will be. So instead of making a texture X = 0.10 and Y = 0.10, scale them up to at least 1.00
Also you should read through this:
http://twhl.co.za/tutorial.php?id=38