In your settings.php file (located in the sites/default folder)
When you go to en/admin/settings/site-information you set the url for your english homepage at 'default front page'. For example node/1
Now go to for example nl/admin/settings/site-information and set the url for your dutch homepage. For example node/2
If you go to /en you 'll end up on node/1 and when you go to /nl you 'll end up on node/2.