Are you asking how to do that in a script in an action or in Photoshop? Also are the two document the same pixel size and resolution?
First make sure the documents have the same print dpi resolution.
A selection and an Alpha channel are basic the same thing so in case you have not created the alpha channel version of your selection use menu Layers>Save Selection and Fill in some Name in the pop up dialog and click OK. Then use Ctrl+D to deselect
Go to the channels palette and highlight the new alpha channel with your saved name. Then drag the channel and drop a copy on your second document.
In the second document use menu Select>Load selection use pull down to select your saved selection name.
In an Action you can not record a drag from one document and a drop on a second document. You need to use some other method. One way is after creating the alpha channel, deselect and highlighting the new alpha channel is to do a Ctrl+A select all Ctrl+C copy the go to the second document channels Palette the click on the add new channel icon in the bottom of the palette followed by Ctrl+V paste.
In a script that for you to do....
Are the two documents the same exact size?
How do they relate (for example name- or path-wise)? This may be important to avoid nonsensical results in case more than two files are open.
2. I want to content aware fill the selection. Is this possible using applescript?