There is one flaw in your post: "popup which is common for both buttons" is not anything certain. Do you mean "the same window object"? If so, it's not a problem: you should have one window variable in outside content: first button (any of the two, whichever comes first) creates window and shows it, the second one just changes content of the same window.
From your post, it's unclear what do you want to do with the window when it is closed and what should cause the window closed. If you don't have to care about preserving some content in the popup window which could be modified by the user, there is no a problem. For example:
<html>
<head>
<script>
var myPopup
function Create() {
myPopup = window.open("", "", "width=200, height=100");
}
function CreateShow(text) {
if (!myPopup)
Create();
else if (myPopup.closed)
Create();
myPopup.document.write(text);
myPopup.document.close();
}
function First() {
CreateShow("first method");
}
function Second() {
CreateShow("second method");
}
</script>
</head>
<body>
<input type="button" value="Show First" onclick="First()"><br/>
<input type="button" value="Show Second" onclick="Second()">
</body>
</html>
If the scenario is going to be more complex, please explain it.
[EDIT]
I also want to warn you: in many scenarios, using popups can be considered pretty bad. The users can be highly annoyed by them, block them by the browser-based plug-in blockers, and so on. This is not because the technique itself is so bad; this is because pop-ups have been heavily abused by shameless advertisers and extremely bad designers; and also because pop-ups take window control out of user's control. Be careful with pop-ups, in many situations, they are best avoided.
—SA