Interact with dynamic loaded swfs

Think that wwe have 2 Swfs. Main.swf including mc_container movieclip to laod other swfs and load.swf , the swf to loaded.

1- If I use lockroot on load.swf, how can I give commands from load.swf to Main.swf ? _root do not work as AS tut says, so whats the command ?

2- How can I send commands to loaded. swf ?
Think that loaded swf has a MC instance named x and a button instance inside x named y.

How can I pass command to that button ?
thanks for replies.