If you ever need to open multiple distinct sessions in the same script and still let PHP generate session ids for you, here is a simple function I came up with (PHP default session handler is assumed):
<?php
/**
 * Switch to or transparently create session with name $name.
 * It can easily be expanded to manage different sessions lifetime.
 */
function session_switch($name = "PHPSESSID") {
 static $created_sessions = array();
 if (session_id() != '') { // if a session is currently opened, close it
 session_write_close();
 }
 session_name($name);
 if (isset($_COOKIE[$name])) { // if a specific session already exists, merge with $created_sessions
 $created_sessions[$name] = $_COOKIE[$name];
 }
 if (isset($created_sessions[$name])) { // if existing session, impersonate it
 session_id($created_sessions[$name]);
 session_start();
 } else { // create new session
 session_start();
 $_SESSION = array(); // empty content before duplicating session file
 // duplicate last session file with new id and current $_SESSION content
 // If this is the first created session, there is nothing to duplicate from and passing true as argument will take care of "creating" only one session file
 session_regenerate_id(empty($created_sessions));
 $created_sessions[$name] = session_id();
 }
}
session_switch("SESSION1");
$_SESSION["key"] = "value1"; // specific to session 1
session_switch("SESSION2");
$_SESSION["key"] = "value2"; // specific to session 2
session_switch("SESSION1");
// back to session 1
// ...
?>

When using this function, session_start() should not be called on its own anymore (can be replaced with a call to session_switch() without argument).
Also remember that session_start() sets a Set-Cookie HTTP header on each call, so if you echo in-between sessions, wrap with ouput buffering.
Note: it's probably rarely a good idea to handle multiple sessions so think again if you think you have a good use for it.
Personally it played its role for some quick patching of legacy code I had to maintain.