Looks like you're right. They're using the same language key. I can make the change for the next build to use separate keys, but if you want to do it yourself in the meantime, do the following:
In /modules/mod_sclogin/tmpl/login.php, search for MOD_SCLOGIN_LOGIN (around line 84).
Change it to MOD_SCLOGIN_LOGIN_MODAL_LABEL. Then in the mod_sclogin.ini define the new string MOD_SCLOGIN_LOGIN_MODAL_LABEL="Are you already registered? Login here" and keep the original MOD_SCLOGIN_LOGIN="Log In"
-Melissa
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