calling admin-ajax.php from admin-ajax.php

Does your server know its own name and/or have loopback connections allowed?

Like, if you do a remote get on just the main blog page, does that work?

If the server doesn’t know its own name because the DNS doesn’t resolve, or loopback connections are disabled at the server level (these are common “security” measures on some cheap hosting), then connecting back to yourself won’t work.

Edit: Okay, so if that is not it, then did you try hooking to the wp_ajax_nopriv_ hook? Since you’re not passing cookies along or any form of credentials with wp_remote_get, then you will be connecting as an unprivileged user, which means it’ll use the nopriv hook instead of the normal priv’d hook.