It'll be fine if you do this in a way that isn't perfect. Namely, by not using jQuery and instead using inline Javascript events.
First, take your link, make it open the content page in the frame if it doesn't already, and then add an onclick that calls a Javascript function. It should look something like this:
<a target="content" href="doc1.htm" onclick="andAlsoDoc2();">
Inside the menu page (not doc1 or doc2) add the Javascript for that function. Its code goes like
<script>
function andAlsoDoc2() {
window.parent.frames["menu2"].location.assign("doc2.htm");
}
</script>
That looks in the parent window (which is the one with the frames), finds the frame named "menu2", and makes it navigate to doc2.htm.
Notice that the function does not "return false": doing so would prevent the original action (ie. browsing to doc1) from happening, and you do want it to happen.