At last! I've managed to show the Facebook and Twitter buttons provided by Simple Facebook Connect and Simple Twitter Connect in the front-end login form generated by Theme My Login.
However, neither the GLOBALS['action'] workaround you suggested, nor declaring global $action in the
function parse_request() (class-theme-my-login.php line 185) did the trick.
Don't know why, maybe it's because of the action queue, but I had to hack sfc-login.php and comment out the
if ($action == 'login') at line 96
In addition, as you remarked here, since SFC hooks to admin_footer and login_footer, I just added to my theme's functions.php
now I'm going to test if everything works as expected